I know you've posed an excellent question to conservatives to get them to actually DEFINE their terms in their repeated rhetoric of "liberal" media bias, but I'd also like to answer with how I think it goes down.
He who has the gold, in this case, the venue of distribution, makes the rules. IF a right wing bias in the "mainstream" media sells well enough (and it does), then the corporate management of all media will continue to keep a right wing bias on the air. The journalists themselves, I believe, while largely fancying themselves either neutral or even - laughably - liberal, follow a pack mentality. They have bought the "liberal media" rhetoric themselves and seek to prove just how "not" liberally biased they are by becoming extremely right wing biased.
All in all, however, it's about what sells and produces the highest profits for majority shareholders. Few controlling shareholders are like Rupert Murdoch, who will throw good money after bad if he has to to see his political vision through. Inevitably is the controlling shareholders of a distributor see too much drop in profit because the VIEWERS left the massive right wing bias behind, then corporate management would begrudgingly change its directions.
Keith Olbermann HAS said that he's fully aware that many individuals in corporate management of both NBC and GE can't stand his views and do not want them on the air, BUT that they will keep to the bottom line if he proves that he can drive up their advertising revenue through ratings. I agree with him. Yet, I also believe that bottom line religion goes just so far depending on the individual in senior management.
For example, I think the only reason Olbermann was ever permitted to editorialize after Katrina is because Jeff Immelt is less ideologically driven than was Jack Welch. I fully believe that Jack Welch would never have allowed the competitive profit opportunity of Olbermann's editorials to arise in the first place. Liberal editorial being so rare on cable news, it was inevitable that Olbermann's ratings would go up and I believe Welch would expected that and not placed himself in a position in which he would have had to explain to shareholders why he demanded cancellation of a show that was on the rise in ratings.