Because 90 major American companies refused to do any advertising during the liberal radio networks broadcasts according to this leaked memo.
Of course, to the left this represents a massive conspiracy between big business, conservative talk radio and Karl Rove, but in reality it probably has more to do with companies not wanting to advertise on radio shows where hosts crack jokes about assassinating the President (among other nasty examples of political rhetoric).
Liberals will probably respond to that by saying that Rush Limbaugh, etc. are every bit as bad as the Air America folks. Something to which I respond: No, theyre not. Rush Limbaugh certainly has his moments. He goes a bit over the line on occasion (like with the Michael J. Fox thing, in my estimation), but almost nobody on the right rises to the level of bile and nastiness on display on Air Americas airwaves.
But then, there could another explanation for this shunning of Air America too. It could just be that these advertisers didnt want to waste their money on radio shows that nobody is listening to.