Okay, it’s a really complicated issue but you’ve got three big things that are all kind of working together here:

1. Political capture: the US’ first past the post electoral system basically guarantees that there’s going to only be two main parties. They were always vulnerable to capture by the wealthy, but the Citizens United decision functionally guaranteed their capture by the groups with the deepest pockets. The democrats themselves are shit scared of any serious left policy because they know it’ll scare off their big donors, and despite the fact that fundraising has not directly translated into winning for them, they’re terrified that they’re going to lose the support of the wealthiest and that’ll guarantee election losses. At least, that’s the optimistic interpretation.

2. Cold war reaction: the US didn’t just have one red scare, we’ve had two or three spread out over several decades. There was a huge cultural reaction against communism after WWII, and being an open communist during the Cold war would just get your ass disappeared (according to my now dead boomer dad, though I’ve seen no evidence to support it), beaten up or killed by locals, or shunned. A lot of folks were terrified of espousing left policies because they could easily be suspected of or painted as communist. While the cold war is passing out of living memory, the chill that it left on American leftism for the better part of 100 years is hard to overstate.

3. Our intelligence agencies have consistently worked across all levels of government (local, state, federal) to harass, discredit, and sometimes kill left leadership and organizations. The CIA itself ran a very successful multi-decade campaign of overthrowing peaceful, democratically elected left-wing governments across the global south by directly sponsoring, aiding, and training right wing reactionary movements, and there’s not really any evidence that they stopped. There’s no reason to think that they’re not still working hard today to prevent any serious left movement.