John Lemon argues that the #2 reason that academics tend to be lefties is that academics are very smart:
Academics, almost by definition, have excelled in schoolwork. That’s how one eventually gets selected to the professorial team. Most academics probably came from the top 5% of their college cohort. They’ve lived a life where they’ve been told they are “super smart” all their lives. I believe this naturally cultivates an intellectual arrogance which makes them believe they can solve any complex problem if they just put their mind to it. Social scientists and humanities professors likely feel they have special insight into the human condition, more so than those not selected into the professoriate, and hence they are tremendous social engineers.
The social engineering mentality tends to be more common among the Left, which sees the role of government to solve social problems. Those on the Right tend to prefer the free market and allow individual citizens to figure out a way around life’s trials and tribulations. This certainly is not a rule that is set in stone, and many conservatives have a social engineering streak when it comes to some social issues (e.g., homosexuality, drugs). (Being more of a libertarian conservative, I tend not to favor such social engineering via the government, but rather prefer the marketplace of morals to duke it out and come up with non-governmental sanctioning of various behaviors.)
While I see some merit in this argument, as well as reason #1, I also agree with commenter John Hudock, who notes that leftism has been especially prevalent on campus since the professoriate got taken over by Vietnam-era draft dodgers.
BTW: The #1 reason, lost in the great blogger archives, is that academics tend not to have any business experience.