Aside from the obvious liberal bias, it is not surprising the MSM keeps writing these stories. First, no doubt, they resent covering McCain's recent "staged" photo-ops. Second, there is probably a desire to balance the "he said, she said" coverage they have had to report on about Hillary and Obama. The problem is, the Democrats are in a competitive race, and the negative things being said about them are organic.
Conversely, the McCain stories (I'm lumping this one in with the "affair" story and the "temper" story) lack context and appear to come out of the blue. They seem contrived, and thus, are barely "newsworthy" enough to be published -- and certainly -- they do not warrant prominent placement in a major newspaper.
This is evidenced by the fact that, so far, these stories have all been one-day stories, with not enough "meat" to generate a follow-up. Real narratives are developed when a story won't go away (think Rev. Wright). The media may continue to manufacture stories about McCain, but without any real substance or evidence, they will continue to fall flat.
Ironically, it may be that the MSM is actually inoculating McCain from future attacks.