Jon Stewart is correct, Ron Paul gets less media coverage than other prominent candidates, at least from mainstream media.
Ron Paul does not get coverage commensurate with his delegate count, voting record, longevity  or influence.
Certainly as a candidate for the Republican nomination that has done as well as he has collecting delegates  he isn't as well known as Rick Santorum or Newt Gingrich, who also ran but didn't do as well in the end collecting delegates.
Ron Paul does not run on a traditional Republican platform and self-describes as a Libertarian running as a Republican .
If you believe that the media has a political agenda then it isn't hard to see that as neither a Democrat nor a Republican there isn't much motivation to give any coverage to what is essentially a third-party candidate (Paul ran as a Libertarian in 1988). This might explain the lack of coverage as the major media outlets cover the candidates they agree with. Libertarian media outlets cover Ron Paul, such as reason.com, but those outlets are much lower in profile.