Protect their investment. Apple can no longer tip their hand to the general community. Any information prior to launch will provide oppertunuty to Cybersquatters and PC rip offs. Sotec being a perfect example. Conserve their Marketing dollars.
News is Advertising, especially now.
Any thunder that is stolen from Apple prior to its launch hurts the launch. Later today there will be several sites posting the spec's on the new iMacs. Apple is weilding more power now that it is more successful. However it has not stepped over it's rights using that power.