Basically, I think, it's because Paul is an antiestablishmentarian with the ability to articulate his beliefs. His ideas would bring real change to the US national state, not just the feelgood, campaign season fog of that hopey changey thingy of which we heard so much last presidential election. The mainstream media is a part of the national establishment even if not officially an arm of government, and like all established institutions its first imperative is self-preservation. Out of that it reacts aggressively toward both people and ideas that pose genuine challenges to its privilege and wealth. Paul is an enemy to the mainstream media because he is an enemy of the status quo.