Extremists move the middle.
Compromise is everywhere. Most of us can't possibly be pure extremists or true fundamentalists, so we draw the line somewhere in the middle.
Consider the choice of what you eat (or don't eat). It ranges from the omnivore at one end to the fruitarian at the other:
Cannibal… chimps… dogs… cats… cows… pigs… foie gras… chickens… fish… unfertilized eggs… honey… yeast… cherries… dust
My guess is that few people care so little about their role in the food chain that they're willing to eat humans (one end of the spectrum), and there are very few strict fruitarians out there (but I've never met someone who wouldn't eat yeast). Most of us draw a line somewhere between the extremes. That means we're already compromising, we just argue about how much.
Karl Marx… Maoist… socialist… progressive… fiscal conservative… libertarian… Ayn Rand
Again, I don't think that many people would be happy at all living at either end of the spectrum above, so we each draw a line. It's ad hoc, it's based on our community, but we pick it and then magically, we stick with it. Not just stick with our ad hoc line, but argue about it, defend it and get angry about it.
Private jet… fried baby seals… SUV…'organic' dry cleaning… Prius… bicycle… localvore… burlap sacks… No impact man
It's interesting to note that an enormous amount of apparently principled argument goes on about relatively tiny movements in where the line is being drawn. In most cases, to paraphrase an old joke, "we've already figured out what sort of girl you are, now we're just arguing about the price." It's not the principle, in fact, it's just the degree of compromise we're comfortable with and content to argue over.
And so it's left to the zealots. The people at either end have little hope of moving the masses all the way to their end of the argument. Instead, what they do is make it feel safer to change the boundaries, safer to recalibrate the compromise. Over time, as the edges feel more palatable, the masses are more likely to be willing to edge their way closer to one edge or another. Successful zealots don't argue to win. They argue to move the goalposts and to make it appear sane to do so.