It's likely that Johnson's crime is that he is too good a candidate with ideas which are too likely to appeal to a broad spectrum of voters and the forces of the status quo in the media and in the party would just as soon not have him challenging their supremacy. They can tolerate Ron Paul because he has no history of accomplishment and can be dismissed as a bit squirrely and academic – he may stir up the rabble but he probably can't win the election and if he did he's such an ideologue he'd be unable to get anything done. What they can't tolerate is is the threat of a younger, more dynamic and less easily marginalized pro-liberty reformer like Johnson. Johnson has a record of successful reform, has no skeletons in his closet and has practical solutions and the ability to be pragmatic enough to get them passed.
Johnson is the real thing, and like Teddy Roosevelt and Barry Goldwater before him, the elites of the media and the party have aligned against him. He's too dangerous to be given a fair chance in a debate, and we've seen a corresponding dearth of coverage in the media when compared to lower polling candidates like Huntsman and Santorum. He is a threat to the status quo cannot be tolerated by the establishment or their media allies.
The debates are grand theatre and the people love their bread and circuses, but as Republicans and as voters we should to be outraged and we ought to demand better.