askchapo

I think it's because party discipline doesn't really exist in the US the way it does elsewhere. In the UK, if Labour drums out its popular elements, they can run off and form new parties and their popularity will travel with them. In the US, if you win, say, a Democratic primary, there's really nothing the Democrats can do to you until the next election cycle, and even then all they can really do is raise a ton of money for your opponent if they don't like you, they can't kick you out in the same way.

So there's a lot more heterodoxy within the two parties, and the alignments of the parties shifts much more over time. The most recent example of this is the Trump takeover of the Republicans, and the looming future of the Republican party being completely overtaken by millennial and zoomer Nazis. It's a far cry from what it was when Newt Gingrich lead the Republican realignment post Reagan/Bush.

In the past, I think the political machines that the parties all relied on lead to a different result because they had different mechanisms of forcing people out that they just don't have right now. The closest we've really seen to the Dems trying to actually enforce a party line was when they censured Ilhan Omar over Israel, a process which just showed how powerless they are in the current state of things to do that.