This article from the New York Times discusses a theory as to why professors are liberal. However, the question to ask about academia is why professors, regardless of their orientation on the spectrum of ideas, generally do not teach conservative ideas alongside liberal ideas. For example, Amherst University has a course entitled "Taking Marx Seriously" but does not balance that with coursework in conservative ideas. Duke University offers a certificate in Marxism and Society but does not offer certificates in free market thought.
Freedom of thought is very important to students, but they cannot make informed decisions about their place on the spectrum of ideas if they are never exposed to all ideas. That is why Young America's Foundation brings big-name conservative speakers to campuses nationwide and provides resources for campus activists who share the ideas of freedom, limited government, strong national defense, and traditional values with their peers.