There are many reasons for this disparity, some are sinister. Others are mundane. Among them is that conservative media outlets hire overwhelmingly more conservative commentators than liberal commentators. Many outlets such as Townhall, part of the Newscorp empire, does not have any liberal commentators and seemingly has not even ever invited a liberal guest commentator.
The mainstream press started at a position where most of their commentators where considered well mainstream. Conservatives complained that this was well somewhat to the left of them. Despite the fact that these columnist are at least as right of mainstream liberal commentators as they were left of mainstream conservative commentators most mainstream outlets responded by hiring Conservative commentators, and encouraging their current commentators to move to the right. Lastly most liberal outlets such as Huffington Post have at least some conservative commentators.
Another reason that conservative commentators are over-represented is the fact that most news media in the United States is add supported, even public radio and television, though they are referred euphemistically as something else. Advertisements are overwhelmingly paid for by businesses. The people that buy the adds dictate the news. This is why there is business section in almost all papers but no labour section, there once was in most papers. The advertisers want to see conservative commentators, and despite claims that editorial policy is independent of advertisers wishes, the advertisers do indeed get what they want.
The final reason I will discuss in this post, is that conservative groups simply buy commentators. Especially if you look at "former liberals", such as Linda Chavez, you will find that they received substantial monetary support from conservative groups before their road to Damascus conversions. Anyone who has studied spy cases will tell you that people will sell out their country, risking execution, for the most nominal of fees. Imagine how much easier it is to get someone to engage in a betrayal that is at least legal.
For all these reasons and many more, there is never likely to be parity between Conservative and Liberal professional commentators. This is why I am trying to start the person to person offense movement. If as many liberals as possible keep records of the various falsehoods and mendacities of conservative commentators and politicians we can provide the resources to allow immediate responses to their arguments despite being out manned amongst professional commentators. Chose your person, stick on them and put your info on the web so we can all use it.