Beats me. But, they are reporting that Hastert will be recommending Dreier (it's not a done deal until the GOP caucus votes) so I'm guessing it's because Hastert expects Dreier to be the most pliant, and to stave off the House conservatives. Before, I would've guessed Roy Blunt would step up if DeLay resigned.
Dreier will be a good placeholder, he seems well liked by all. I'm sure Blunt discussed it, but he probably wants a fair shot at the position for a full term after the 2006 election.