Why? Because most Repub pols signed Grover Norquist's 2009 pledge never to raise taxes...why would a Repub pol sell his/her soul to GN? Because not raising taxes helps the most wealthy among us, and they donate to pol campaigns more often and in larger amounts than, say, you/me.
Two Repub exceptions I can think of off hand who didn't go along with GN are Maine's Sen Susan Collins, who said the only pledge she takes is to uphold the Constitution...and Oklahoma's Sen Tom Coburn, who recently said it was "stupid". I read somewhere there are seven Repub senators who have not signed GN's pledge, but I know only these two names.
Interestingly enough, Repub candidate for president Jon Huntsman has rejected Grover Norquist's tax pledge...perhaps that's why we don't hear much from Huntsman's campaign.
Early Repub front-runner Mitt Romney has agreed to sell his soul to GN and will sign his pledge.