I think what happened is back in 2000, the media loved McCain because he was "Anti-Bush"...then they loved him all along until they realized "Wait a minute...he's going to be a REPUBLICAN President" and turned coat.
There is also the significant fact that McCain changed beyond recognition between 2000 and 2008 - first he became one of the greatest warmongers in the Senate, then ditched all of his moderate positions to endear himself to social conservatives.
In any case, the media likes underdogs, especially when they're principled and honest, which McCain was in 2000 and Bush most certainly wasn't.