Seems antithetical to proper critical thinking.
It is, but there's a subtle culture within academia that associates left/Democrat with intelligence, and a less subtle culture that favors the party that supports giving more money to education.
Try telling a professor that you watch Fox News to understand popular thinking and they might respect that, but only after smirking and laughing a little in a condescending way. Then try saying that there are actually some things that Fox gets right and they will get more serious and agree with you a bit, but simultanously express a sense that Fox saying something valuable is an exception, implying that the rest of what Fox says is total garbage.
Now try the ultimate leftist blasphemy: tell the professor that you actually think Fox News is a better news source overall and that CNN etc. are more politically biased. At this point, the conversation just stops and you have been placed on the professor's "no longer take serious" list. You've just blacklisted yourself from recommendations, promotions, etc. unless the person sees helping you as an opportunity to set you straight from your delusion by getting you into some institution or job where you will be corrected.
Now think about all the people who have been blacklisted for choosing the wrong party and now go back and answer your own question of why college professors lean/teach left.