Because they're lazy and don't back up their claims.
Please read former Bush adviser, and Duke prof, Peter Feaver for the latest proof. He writes that the press has been too easy and Obama (i.e. "worship") and was way too tough on Bush. (No, seriously.) Weaver certainly has every right to push that spin. But if you're going to critique the press it's usually a good idea to, y'know, provide some examples/proof to substantiate your work.
But Feaver, like so many who whine about 'biased' press coverage, can't be bothered with specifics to substantiate his claim. (i.e. actual media citations) Zero. Nada. Zilch. Readers are just supposed to trust him.
Instead, Feaver makes lots of sweeping generalizations:
Clearly the mainstream media has not yet figured out how to cover Obama. I don't expect them to subject the Obama team to the same kind of tendentious and mocking ridicule that was the norm for so much of the Bush coverage, but nor do I expect the current prevailing double-standard to persist throughout his entire tenure. The media needs to figure out how to live up to their much-heralded (by them) watchdog role, because the media serves an essential function in maintaining a functional marketplace of ideas. When the media shirks its traditional role as skeptical truth-squadder the way it has shirked during Obama's first 100 days, public debate and public understanding of the critical issues of the day suffers.
Are you laughing out loud that a a former Bush aide is complaining about the press walking away from its watchdog role? Here's five (six?) words for Feaver: run-up to the Iraq war.