From an article in the Chronicle:

They found that in a variety of ways, conservative students were less interested than liberals in subject matter that often leads to doctoral degrees, and less interested in doing the kinds of things that professors spend their time doing.

For example, liberal students reported valuing intellectual freedom, creativity, and the chance to write original work and make a theoretical contribution to science. They outnumbered conservative students two to one in the humanities and social sciences — which are among the fields most likely to produce interest in doctoral study. Conservative students, however, put more value on personal achievement and orderliness, and on practical professions, like accounting and computer science, that could earn them lots of money.