Mostly because the Democrats had no ideas, no plans, no platform, no leadership, and no scruples. When Bush was re-elected, everybody knew he'd do *something*, even if nobody knew what the hell it would be -- to paraphrase Will Rogers, even if he'd burned the White House down, we'd have said, "well, at least he got a fire started."
It's all well and good to complain about what the guy in office is doing, but if you're not offering an alternative, then you just look like a whiny fool. John Kerry's entire campaign consisted of him looking like a whiny fool. After all was said and done, calling Bush names just wasn't enough to get him enough votes to win.
Also, the Democrats -- who tend to arrogant beyond belief -- thought that they could insult half the population, calling them stupid rednecks, and still win the election. Would you vote for someone whose supporters have vocal and dripping contempt for you, saying in effect "I'm far superior and much smarter than you, so obviously you will vote as I say you should"?
Neither would most Americans, as the Democrats learned the hard way in '04.
Now watch this. Most of the responses from Democrats will be something like, "Because Southerners are all idiots" or "Because most Americans are fools" or something like that. And they'll lose again in '08, and still be scratching their heads wondering why. It's sort of funny, when you think about it.