Three reasons (in my humble opinion):
1. Ratings - there are simply too many candidates, and the media is going to focus on the frontrunners. Same happended to Bill Richardson, et al, on the Democratic side.
2. The Two-Party System - I like Ron Paul, but he doesn't belong in that bunch. He is too extreme for most Republicans. He's also too Republican for most Democrats. If there were a third party, he would flourish.
3. The Format of the Debates - let's be real...how can any candidate explain himself/herself in 90-second clips? This format is a joke, we never get past the superficial. And I don't mean this as a criticism to Ron Paul, but in 90 seconds he really never gets past his point about Iraq. As a result, no matter what the question is, Ron never gets past his "first we need to get out of Iraq." For people who go the extra step and search online, they get to hear all of his stances on the issues. But in 90 seconds, you can't get deeper than that.