Old news: humans regard morality as though with a ‘moral thermostat‘.
…we propose a framework suggesting that moral (or immoral) behavior can result from an internal balancing of moral self-worth and the cost inherent in altruistic behavior. In Experiment 1, participants were asked to write a self-relevant story containing words referring to either positive or negative traits. Participants who wrote a story referring to the positive traits donated one fifth as much as those who wrote a story referring to the negative traits. In Experiment 2, we showed that this effect was due specifically to a change in the self-concept. In Experiment 3, we replicated these findings and extended them to cooperative behavior in environmental decision making. We suggest that affirming a moral identity leads people to feel licensed to act immorally. However, when moral identity is threatened, moral behavior is a means to regain some lost self-worth.
This doesn’t appear to always hold though. Most people oscillate happily around a normal level of virtue, eating more salad if they shouted at their child and so on, but some seem to throw consistent effort at particular moral issues, or make firm principles and stick to them.
It seems to me that there are two kinds of moral issues; obligatory and virtuous. Obligatory things include not killing people, wearing clothes in the right places, doing whatever specific duties to god/s you will be eternally tortured for neglecting. Virtuous issues make you feel good and affect your reputation. Doing favours, giving to charities, excercising, eating healthy food, buying environmentally friendly, getting up early, being tidy, offering to wash up, cycling to work. Outside of these two categories there are what I will call ‘practical issues’. These don’t feel related to virtue at all: how to transport a new sofa home, what time to have dinner tonight, which brand of internet to get.
‘Moral thermostat’ behaviour only applies to the virtuous moral behaviours. The obligatory ones and the practical ones demand exactly as much effort as they take, or you feel like putting in respectively. The people who pour effort into specific issues are mostly those who are persuaded that the issue is an obligatory one or a practical one. A clear example of those who push a moral issue into the territory of obligation is of vegetarians, for whatever reason.
Meteuphoric 
Here’s a hypothesis.
Obligatory morality serves, in addition to its practical coordinating function, to distinguish in-groups and out-groups.
Virtuous morality acts as a costly signal for the fitness of an individual within a particular group.
Here is a prediction:
The moral thermostat applies to obligatory morality when cultures clash.
Here is an interpretation of some data supporting the prediction:
Moral crusaders are losers within a larger mainstream culture striking out to found a new moral baseline to elevate their subculture. Because they don’t already feel accepted as part of the mainstream culture in which they reside, their moral thermostat detects low self-concept. But this low self-concept is not due to low status within the mainstream culture per se, but do the low status (not necessarily due to morality) of their subculture within that mainstream culture. So their moral thermostat directs their effort toward putting on a show of elevating a piece of morality which their culture happens to have (or finds it easy to adopt for these purposes), and which it hence considers obligatory morality, to the status obligatory morality within the mainstream culture. This happens because what is lacking here is the status of the subculture, not the status of the individual per se. And what this does is to elevate the status of the subculture. There is a seeming paradox: if say vegetarianism were universally adopted, then vegetarianism as a culture would not be able to distinguish itself. The same applies to individual vegans: if everybody is vegan, they are no better. But hypocrisy saves the day, because the goal is not to win the war, but to distinguish oneself with (and within!) it. In the context of a culture war then, a piece of obligatory morality can function as virtuous morality.
The same thing happens in international diplomacy. With environmental or labor standards for instance. Where what each nation considers obligatory morality they employ as virtuous morality to lift the image of their nation.
So, apologies for the length of this comment, but relating back to your post: Those few individuals who seem to put consistent effort in a particular issue, may simply be fighting a larger war, and the underlying drive is still the same: their moral thermostat detected an internal imbalance in self-concept.