Here's Tim Stanley from the Telegraph with a pretty good explanation why:
Sometimes the media ignores Ron Paul out of ignorance or spite. Sometimes it’s because they’ve done the math and convinced themselves that he can’t win. Other times, I suspect that they take a look at the people chanting his name in the bleachers and they don’t like what they see. The Paulites are good folk worried about the direction of their country, but their exclusion from the mainstream makes them come off like the vanguard of a Hicksville revolution. Television is a cool medium. Ron Paul and his angry army are too hot for it.Update: But Ron Paul's not frustrated. From Dr. Paul:
it’s not like I’m just trying to win and get elected. I’m trying to change the course of history.