What is it about the US political system that forces things back towards a evenly split equilibrium?

I don't agree with the typical "vendors on a beach" (move towards the center until you each grab the half that's closer to you) metaphor since it seems to discount the possibility of being so far away from people that they're alienated to the point of non-participation. I could see it being cyclic though between that and periods of recapturing alienated demographics.