Because the left believe they've completed taking over institutions (except SCOTUS). US presidential

elections result is not as important to the left as to the right. Indeed. The right put all their hope on presidential election outcome. If they lose presidential election, they will have nothing in power (again, except SCOTUS). But the left, if losing presidential election, will still have education, media, legal... all these institutions they controll, which as a whole is more important than president alone. So, the left dare to make unpopular policies even if they lose presidential election, but the right don't. The right don't have the luxury that the left have.

The root of all these can be traced back to a generation ago, i.e. 25-30 years ago when the left took over the most important institution: education.