Nominally, he withdrew because of tax questions, but there are reports that this New York Times OP/ED convinced him to withdraw.

When even the New York Times realizes that you have functioned as an über lobbyist and calls you on it as game over.

Matt Taibbi was far more colorful in his description of Daschle and his revolving door porfiteering, “Tom Daschle would suck off a corpse for a cheeseburger.”