Because what if there weren't a "liberal" bias - conservatism NEEDS liberalism in like air. It can't function without it. Because it keeps people in "the middle" from going too far from the norms.
Put some mildly liberal people on television or radio and call them radical liberals and you contain the conversation - the body politic itself - exists within. Occasionally you run across ACTUAL liberals - be they foreigners or intellectuals or whatever who have independently arrived at a liberal point of view.
But what happens by having a "liberal" media is that the discourse in contained - perhaps that's good - perhaps not.
Besides if you start to REALLY examine how profoundly f*cked the middle class is getting by both parties in the US, in particular - you realize that it really is about the very wealthiest 1% or so, versus everyone else - and guess what - as long as you perceive it as something other than that - their victory is achieved.
If you don't believe me - take a challenge - listen to WBAI on the web or on the air - it's partially Pacifica Radio - but it also for some of it's time hosts local (self-described) socialists from Brooklyn.
Listen for a few afternoons and see how ACTUAL self-described socialists discuss matters - or what they discuss - and it becomes painfully clear, that whatever our media in the US is - it isn't particularly liberal.