Because only 2 have any chance of winning. itd be like if the media gave the same level of coverage to an independent movies release that it gives The Dark Knight Rises. One is a series in a known franchise, has a studio behind it, lots of money behind it and millions of fans actively participating. The other might deserve a little mention, but it hasn’t been vetted by any process to enter it into the mainstream, hasnt achieved a level of success to warrant coverage equal to movies/nominess of the major parties who have, isnt even a quarter as widely known and it has no chance of beating the competition its being compared to.
Ron Paul fans especially like to make the claim that he is being shut out of the media, which is ironic because they’re libertarian yet are asking for Affirmative Action for their candidate.