chiefly because he had less baggage than the other republicans:
1) Fred Thompson, had been running a lackluster and unenthusiastic campaign for months, when critized by the media for just that, caugh on camera explanining ( i'm paraphrasing ) "I don't particularly feel like running for president". Stick a fork in him.
2) Huckabee. Goes on into a debate saying he's the only one on stage with a theology degree. He doesn't have a theology degree. Also caught lying about how he raised taxes in arkansas.
3) Romney. Basically a social liberal who changed all of his positions within the last 2 years. Flip flopped on abortion, gay marriage, immigration, etc, and is now campaigning as a red meat republican despite having a very liberal record on everything. Is a mormon, and something like 25% of voters say they'd never vote for a mormon, and 63% of voters say they'd never vote for romney in a poll ( the highest of any of the candidates this season ). Also he's made of plastic.
4) Rudy Giuliani. Pro-Choice, twice divorced, his own kids are not on speaking relations with him, one of which is supporting barack obama. Caught using government money to fund his extramarital affair, with a security detail for his mistress. When asked about the expenses by investigators, said it was security related. Also was a complete idiot to only campagin in florida and ignored the earlier primaries.
5) McCain. Hasn't run a particularly inspiring campaign but easily beats all 4 of the above idiots.