I asked this question on a recent 'question day' that Jonathan Bernstein hosted. His answer is that forces within a political party are more often centrifugal (fleeing the center and moving toward one side). People get involved in parties because they really want a particular policy on an important issue. They are passionate, and they pull the party that way. The non-passionate aren't involved, don't vote in primaries, and show up only at the end--at the general election. Then the moderates are disappointed with the choices, but they weren't involved in earlier decision-making, so maybe they shouldn't be complaining.
It's true. I'm a member of a political party, but I'm not exerting myself in an attempt to pull the party in my direction. I suppose I shouldn't complain, but I still will. Maybe I'll start thinking about how to get involved in my party and push them in my direction.
Luckily, there are moderating forces that act on parties. Hooray!!! Nate Silver writes that parties become more moderate in their candidate selections after they've lost a few too many election. Yes, they wise up, stop listening to those passionate wackos, and choose someone less frightening to the center. Why does it take a bunch of losses? Why are the lessons unlearned after a few wins? I don't unlearn lessons.
Or maybe I do... Perhaps, when things are going well, I forget some of those humbling lessons, like not to be troll, or don't overestimate my value to my employer.
Perhaps I'm starting to understand why parties are the way they are. But I still object.