This New York Times article reviews new research into why the Academy leans left. The researchers' conclusion: typecasting, which influences a young person's idea about what they want to be. The article reviews some other recent theories, also.
While I'm sure "typecasting" influences some, I'm waiting for someone to research the numbers of highly qualified academics who consider themselves libertarians, classical liberals, or economic conservatives and have not been able to get jobs in Academia. I know far too many, and their stories, for it to be a fluke.