Well, other than it was a polite thing to do, it probably has to do with the fact that we like to win races. Choosing Governor Dean is likely to prove to be a disaster of about the magnitude of nominating Senator McGovern back in 1972.
Maybe even worse -- Governor Dean is at least borderline mentally unstable, so perhaps it closer to the Muskie or Eagleton contretemps than McGovern. Whichever, it is all to our good.
They could easily have chosen someone far more in the center from the DLC and let the DLC rebuild the party, but they decided for the ideological purity of yet another northeastern liberal.
"The polite thing to do" is not normally "the smart thing to do" politically. We need to get over this hangup.
Mehlman's failing here was to make a comment that completely papers over Dean's vicious extremism. In politics, you can't assume that "people will get it." You need to help them understand, using EVERY POSSIBLE opportunity to TEACH them. Mehlman's stupid comment ratifies Dean as a legitimate political opponent, when in fact the man is a hate-filled extremist. Our RNC chairman failed to call it like he sees it, usually a mistake in politics.