Because Bush promised to put good solid conservatives of the breed of Scalia and Thomas on the Court. He did not compromise on court appointments during his first term, preferring to endure the filibuster, so it looked like a good bet to vote for him again in 2004 on the basis of these promises. That was also the reason why people worked so hard to increase his majority in the Senate.
Now he has broken his promise. It's not the first time that a politician has done so, but I will say that Bush is usually a man of his word, which makes it all the more disappointing. Maybe he thought he was doing the right thing by nominating Miers, but now he needs to think again.
I'm also beginning to think the Republican leadership has been playing Three Card Monty with the filibuster smasher. And by that, I mean it's been intentional.