Because that cowardly scumbag Bush is already assisting the Hillary campaign with the transition.
Sen. Clinton has a way of getting senior men of both parties to do her work for her. Pre-emptive removal of Gen. Pace excuses candidate Clinton of the responsibility to vote for or against re-confirmation. As a bonus, Hillary and other feminist senators get to vote for a liberal admiral, CNO Mike Mullen, an ardent advocate of diversity quotas and other controversial goals for the military.
It has long appeared that Bush is doing everything within his power to pave the way for a Hillary Presidency. Those new bosom buddies Poppy and the Slickster are undoubtedly pleased.