Because the beltway press corps has conditioned itself to respond only to Republicans. They've trained themselves not take Democrats seriously, either the rank and file who inconvenience them with e-mails they do not want to read, or the leadership they simply disdain. Unpopularity obligates them to criticize Bush at least mildly, but the relief they feel when his numbers edge up a bit is palpable. They don't seem to know this about themselves.
This really is true. The press really is unable to hear and comprehend press criticism which comes from the left. It's like they're unable to process it. They love parading Bernie Goldberg around endlessly so he can tell them how evil and horribly liberal and rotten they are but will then completely ignore a book like "What Liberal Media?" which had the advantage of containing actual research and not being utterly incoherent.