Even if all the Republicans in the senate stuck together, they couldn't get the 60 votes to get it repealed. Also there were quite a few RINOs who voted for Motor Voter. I'm sure quite a few RINOs would oppose its repeal.
True, that must have been how it passed in the first place.
Still you never saw the Republican House under GWB try anything like that by sticking it in other bills (as the current house fights the Democrat Senate.) Instead Tom Delay and Bush went for the ‘buy voters’ short term strategy.
Worse yet they simply re-authorized the Voting Rights Act in Fall of 2006 with NO changes to get it out of the way, and lost minority voters anyway.