Politics. Hastert and the boys are upset and will be even more negative towards any of Bush's desires, especially immigration related, if they are forced to hand over the documents. I mean they even threatened to cut the FBI's budget (a first for congress, actually cutting something)
On other hand, Gonzales, the head of the FBI, and the second in command at the DOJ have all threatened to quit if the documents are returned to the congress.
The 45 day cooling down period is to provide room for Hastert and Gonzales to find a solution acceptable to each. In essence, giving some fig leaf to Hastert.
The 45-day cooling off period also gets into the SUMMER Session where CONGRESS goes on Vacation until Labor Day!
I wonder how this plays into the scheme of things.