When Democrats are in power, the Republicans can stall things because they only have a slim majority and the extreme measures that they’d need to take are unpalatable to 5-30 of the most regressive Democrats. So their majority doesn’t count for a lot.

Overall, I think this is partially just that Americans are pretty regressive (possibly because of all the propaganda, possibly because of our poor education). And partially the successful efforts by Republicans to control local government, which allows them to do things like gerrymander federal districts.