The obvious reason, of course, is the unpardonable pardon of Marc Rich.
A blogger at the liberal DailyKos writes:
"Two days ago, knowing that there was a Karl Rove-led effort to "go after" Attorney General nominee Eric Holder, Patrick Leahy inexplicably agreed to a Republican, request for more time to "prepare" for Holder's confirmation hearing."... What might explain why Leahy "inexplicably" did this? Don't forget, Leahy was very critical of the Marc Rich pardon...