Mostly because you can't play around with the absolutes of math and physics, there's no room for interpretation--so a prof can't explain a math theorum or something within marxist or radical terms. However, in the humanities, weak minds abound. More and more, students just skate by on feelings and unfounded observations.
In the Creative Writing department of my school, there were only TWO deeply closeted conservatives (myself, and my best friend). Our fiction was one thing--we could actually float by some really subversive, libertarian ideas amid the standard, liberal drek, but we found it very difficult to stand out and make our own statements in the literature classes that were part of our 'foundation' courses.
Still, and I say this with a great amount of shame, there was such a hatred of anything conservative, that we often just kept our mouths shut in class, at the department cocktail parties, etc.. The surest way to kill a book deal or a possible meeting with an agent or editor is to say, "Ronald Reagan was the GREATEST PRESIDENT EVER!" or something.
Ever heard of Lysenkoism? Crippled Soviet science for decades.