I think it's mostly because true political change is messy. People are self-centered and if they are doing okay, they will prefer stability to revolution almost without fail, regardless of how rotten the system is or how many people it cheats and abuses. It's why the Sunni merchant class in Aleppo supported Assad just about to the point he started dropping bombs on their houses. Real change in America would mean short term strife and uncertainty, even if it vastly improves long term prospects, and there's always the risk that the attempt will fail and things will end up worse than before. That's not a trade that well-off people are historically keen to make, so they stick with the devils they know, quibbling over their relatively minor differences. Real change requires a desperation strong enough to break the seal on pandora's box through a willing rupture of the status quo.