It sounds like I'm just making a partisan shitpost but it's based on very simple logic
Conservatives have to have a generally positive view of America because they want to conserve it. They generally do not want to change things and when they do, it is changing things to how they were previously, thus being called backwards or regressive.
Leftists have to have a more negative view of America because they want to change it. They focus on flaws thus they think we need to fix institutional racism, gender equality, transphobia whatever etc. they think America is not good enough, since they want "progress" aka change
Note how it is very rare for the right to say "I wish we were more like x country." But the left often says we should be more like Sweden or Nordic socialist countries.
Here is a short explanation that goes deeper on it
Most leftists think that we are racist, intolerant, xenophobic and bigoted. Thus they want to fundamentally destroy institutions and replace them