There is considerable disagreement over which things are roughly good or bad for the world. Encouraging same-sex marriage, speeding the cogs of ‘progress’, compelling would-be immigrants to live in third world shanty towns, and building self-replicating nanobots are considered by different parties to be either substantially positive or substantially negative. Such questions are sometimes loudly and fiercely debated.
There also seems to be much disagreement over how good different good interventions are. Is promoting sustainable agriculture a crucial last-ditch countermeasure in the face of impending societal destruction, or harmless feel-goodery? This type of disagreement seems very widespread, given the huge variety of endeavors people support with their money and time. However such disagreements are much quieter; for the most part people politely get on with their own causes. You are much more likely to be handed a brochure explaining the virtues of buying beads from women in the developing world than one earnestly arguing that this isn’t particularly valuable.
This seems like some evidence of a tragedy of the commons that one might expect prima facie. Even if you can hand out don’t-expend-effort-on-veganism brochures more cost-effectively than the vegans can hand out theirs, you can’t capture the efforts of the would-be vegans for your own preferred cause. The efforts you divert will probably be spread across many other causes.
Ultimately then, the public discussion of causes should become full of hooks to harvest goodwill and efforts, with few working to rescue people from bad causes and replenish the stock. The main demand for counterarguments should come from the people who wish to avoid joining causes themselves. But they don’t need strong cases, just some excuse to say as they decline to join. And indeed, the retorts used for such purposes seem rarely strong enough to change a person’s mind.
One big exception to this tragedy of the commons situation would be causes where people are quite divided – where leaving the pro-X side means joining the anti-X side, and where the anti-X supporters would be happy enough to diminish the X side anyway. These causes tend to be of the first sort I mentioned: where the disagreement is over whether X is excellent or hellish, rather than good or better. For these issues there are no tragedies of the commons, and we should expect to see elaborate cases made for both sides. As of course we do.
Another situation where you wouldn’t see this behavior much is one where most of the causes are considered similarly good by most people, but there are still a few much worse ones. Then a person who understands this state of affairs can redirect most of the efforts they divert from the bad causes toward causes that seem best to them. So if the tragedy of the commons account is right, it seems we are not in that situation. People either think a relatively narrow range of causes are quite good, or that so many causes are quite good there aren’t bad ones around to divert people from.
I don’t know that this is most of the explanation, though it seems it must happen to some degree. People may also prefer not to speak against anything that seems ‘good’ – and measure good against some consistent zero point, not against the opportunity cost. It could also be that the easiest way to compare things is for different parties to make a pro- case for each, then to compare those. Though it seems in practice that would leave out many negative details about each cause. Any other theories? Any reasons to believe or disbelieve this one? Note that the heroes of this story would include the selfless people who go out on the internet night after night, with little to gain by it, tirelessly assaulting any position that shows weakness.
Meteuphoric 
People also aren’t necessarily trying to do “good” and working within a framework where objective/intersubjective comparisons of “good” are always possible, rather than supporting causes that captured their interest for various personal, or highly scope-insensitive, reasons.
Might be worth editing to use the more precise term “same-sex marriage” – no-one minds homosexual people getting married so long as they do so with someone of the other gender.
I’d guess that in general it just looks bad to bad-mouth things that are good relative to some zero-point. Forager norms are like this. They complain against you if you don’t contribute food to the band, but not if you don’t contribute maximal food.
Here I will analyze your post and apply your Tragedy of the Commons approach and give my own spin on the same problem…
Tragedy of the Commons – terminology:
commons = available people to become attached to a cause (a potential follower)
causes = organizations in search of followers
cause-hooks = leaflets being handed out (or whatever) to recruit new followers
action = making a donation, marching in protest, etc. (followers doing something to grow the cause’s power thus making the ideology be more likely to be implemented)
My analysis of today’s world using Tragedy of the Commons:
Each cause attempts maximum exploitation of available unclaimed people (the commons) as followers. The result is an overexposure of cause-hooks to people, causing the commons to become unresponsive to additional cause-hooks. People have a limited attention span for accepting new cause-hooks. Result: People become apathetic and contribute action to no or few causes.
[I have experienced this in my own life. I contributed to a few causes and then started ignoring all cause-hooks after receiving too many requests for donations from related-but-different organizations.]
Regarding your “where people are quite divided” problem: I think a person can feel strongly about an issue but do nothing about it. The cause does not benefit by people in the commons who take no action to become followers. I personally have a lot of strong beliefs that I’ve never spoken to anyone about, due to lack of time or interest in discussing with anyone.
You say there is no tragedy of the commons in cases “where people are quite divided.” I disagree: There may be no tragedy of the commons in the sense that the commons are “ideas-being-discussed-at-detail” (your view of what the “commons” are I think) but there is a tragedy of the commons if the commons are “people-who-could-potentially-be-recruited” (my view).
It depends on how you define commons. I am defining it as equal to “people taking action” rather than “people analyzing” or “people debating” about it. For a cause, in my opinion, words are only useful if they create action (thus implementing a social change).
Regarding your “most of the causes are considered similarly good by most people” case — where a person has a kind of equal contribution to many different unrelated causes. I believe this doesn’t happen that often just due to the capacity a single person has for attention. Logically perhaps it would be best for a person to analyze every potential cause and contribute a correct weighting of action to each based on their belief system. However, I think people don’t have time to do that amount of analysis of causes, so they just pick one (or none at all due to information overload/tragedy of the commons).
I don’t think in terms of bad cause / good cause. Rather than viewing each cause as a single entity with two poles, pro- and anti-, I think it would be better to think of them as independent organization-entities. E.g. Both pro- and anti-abortion organizations exist but I philosophically think of them in terms of groups of humans rather than ideas to be debated. In other words, the cause ideology is actually subordinate to the organization. Without the organization, the cause wouldn’t exist (because there would be no one to spread the word).
Finally (sorry this reply is so long), I think your heroes might be wasting their efforts. The way for a organization to grow is to speak to non-converts, not just amongst themselves. Also it is pointless to debate with the opposing-point-of-view-organization, whose members are the least likely to change their minds in comparison to unaffiliated people.
Unfortunately in this world, strength in numbers of followers is more important to the success of an ideology than is the validity of the ideology itself.
P.S. I also agree with Nick T above — my reply is just a longer version of his really. Additionally I found the book “Rules for Radicals” helpful to understand political organizations.
This isn’t a failure of the marketplace of ideas specifically but an expression of a general failure of the market: market competition is biased against negative information about goods and services. That’s why the state must require tobacco companies to put warning labels on their packages—we don’t rely on the market inducing rivals to supply the negative information, even if its in their competitive advantage. Or a television commercial tells you how you’ll be so sexy if you buy a certain sporty vehicle; there’s nobody paying for commercials to tell you it’s not true.
The analogy between the two kinds of markets makes clear that your diagnosis is correct: no competitor stands to profit enough from dissing a product when the users might just go to another competitor. Unlike Robin, I see no reason to invoke “forager norms” when the same effects can be observed if the product is not popularly regarded as “good”: cigarettes.
That the tragedy of the commons applies to negative arguments concerning “projects” is simply your most important idea yet.
But note the tension between it and another idea you and Robin promote: that commercial advertising isn’t obnoxious (not to put too fine a point on it). Advertising is probably a main vehicle by which people are drawn to purchases without adequate consideration of negative information. (Of course, the institution would require replacement rather than simple elimination.)