because, unlike individual voters who get to pick who represents us, the Republican party is stuck with the people the voters pick in each state. And the senatorial committee exists NOT to make the republican party into the pure party we want, but to get republican senators re-elected.
We don't have to give them money (I don't). We can scream about their negative ads (I did).
But we can't expect them to try to dump one of their own for a challenger, even if WE like the challenger.
This doesn't really apply to Bush -- although Bush probably promised to support Specter in 2002 in exchange for Specter's support in the 2000 election (for whatever good that was, since Bush lost Pa. anyway).