I’ve often heard that so many in the media are liberal, because they want to work for change, they want to “fix” society. Their worldview is that our society and culture are full of problems, which need “fixing”, and that they can contribute by being “reporters” or cheerleaders for liberal causes.
I recall a survey some years back, which showed that 89% of media members voted for John Kerry. In an election which Bush won by 51-49%, clearly seeing an 89-11% split in the media showed that the media are not any sort of cross section of society.
On a related note, not too many conservative people think of themselves as wanting to be crusaders for the cause and all that entails, so then relatively fewer conservatives tend to go to J school and become reporters.