The basic business model of a newspaper (and a commercial TV channel) is to sell advertising space. Once you fix that firmly in your mind things make a lot more sense.
Newspapers require journalists to write stories that will make people buy papers. Sometimes they might be some public good (Watergate). Often times it will be utter drivel (almost anything about Britney Spears). As long as the stories fit the business model (serious investigative reporting, celebrity trash and anything inbetween) then it's all good.
It gives the 4th estate too much credit to think they are particulary political. Fox hammer the conservative message because their business model is to attract conservative politics lovers/haters to Fox. Their perceived politics is just part of the business plan.
Obama is popular - therefore newsworthy - so stories about him will sell.
McCain is less popular and in many respects less newsworthy (his politics and personal life are well known now, and wealthy old white male politicians aren't scarce on the ground) so stories are not going to sell as many papers.
Long story short - media don't exist to help voters make informed choices; they exist to make money. That ambitious politicians and their campaign teams use this to their advantage should also not be a surprise.
FORTUNE'S FUN FACTS TO KNOW AND TELL: A firefly is not a fly, but a beetle.