A couple of things to point out:
Talk radio IS predominantly right-wing. As was rightly observed, talk radio rose as a counter-point to the mainstream media. And indeed, a lot of the current crop, from Michael Regan to Ollie North to G. Gordon Liddy, are attempts to ride Rush Limbaugh's coat-tails.
In fact, however, listen to Rush for even a little while and you will hear him acknowledge that he is, first and foremost, an entertainer. And let's face it, the right wing talk shows just do a better job of entertaining than the few liberal ones (e.g., Alan Coombs, Bill Press) do.
Talk radio is hardly "mainstream media" though. Rush, et. al. are primarily preaching to the choir. (And I admit, I'm in the choir!) On the other hand, the majority of Americans are getting their news and analysis from sources that are decidedly liberal. (Or, in the case of public television, socialist.) The list of liberal media runs the gamut from CNN to Time-Warner to papers like the NY Times, L.A. Times and Washington Post. Sure, you have counter-balances like the Washington Star and Wall St. Journal. But let's face it, most Americans aren't reading the WSJ!