Because there are two Republican parties in Kansas, and Moore has built a very successful coalition of Democrats, independents who would rather puke than vote for anyone from the Phill Kline branch of the party, and moderate Republicans.
Moore has benefited from the fact that he has spent most of his time in office under a Republican president. He's been able to chart a moderate course and vote with Bush maybe a third of the time on less important issues. But now he's stuck with Obama and he's been forced to make some votes that aren't overly popular around here. He voted for cap-and-trade, for card check, and may be forced to vote for Obama's hell-th care plan. That won't sit well around here. So for the first time in 10 years or so the GOP has a chance. The right candidate will knock him off.
I’m rather surprised he hasn’t attempted to make a move up by now, to Governor or Senator (Governor being a better bet, since the Dems have far better luck with winning that office). After all, how long does Carol Marinovich intend on waiting for Moore to retire ?