Premise: That people with more education are more likely to be left-wing and liberal. This seems to be the case. Why?
I can't accept that it's that education exposes a person to more cases and because of this they develop a greater sense of social justice and a smaller emphasis on self. There are so many people without a formal education but with experience ranging far wider than any academic. I also don't believe that it's that somehow people are taught out of right-wing ideas into left-wing ones. Both ends of the political spectrum are valid answers to the governmental problem; why should education - of any but the most specific kind - enable people to see that a right-wing answer is somehow simplistic and that liberalism solves the hidden complexities? Both right and left wing stand up to solid debate and academic rigour. Politics, instead, is a gut feeling.
No, I think that knowledge is a meme, and memes want to spread. You never see an educated person wishing that they didn't know so much, or that they'd never learned. Knowledge carries with it the spirit that knowledge is intrinsically good, and that other people should also be educated. And if a person makes a political judgement with that in mind, perhaps they're more likely to conclude that left-wing policies would best promote that kind of spread of knowledge they would like.