Well, there are a couple of reasons.
Education opens your mind to the complexities of life, to the fact that there are few blacks and whites in the world, only many shades of gray. In the process of earning a PhD -- a requirement for becoming a professor -- and conducting our research, many of us travel extensively, we are exposed to a wide variety of people, cultures, points of view. That experience genuinely broadens the mind and develops intellectual and cultural sophistication that is simply not compatible with narrow-minded conservatism. The fact of the matter is that there ARE conservative professors, but they tend to be LESS conservative than conservatives in other walks of life. That is because the experience of academic life and research militates against that kind of often-rigid and reactionary conservatism.
The second reason is that the academic life tends to attract more liberal-minded individuals in the first place. It's the kind of career that simply appeals to the already well-educated and well-read, people who are interested in research, in complexities both scientific and cultural, in studying the past or improving the future via teaching or your own work. So, your average entering class of PhD students in any department is going to include a fair number of liberals.
Though as I said, there are a fair number of conservative academics, too.