Because the "normal" US timezones sys admins will be on duty at that point. When folks try to 'slashdot' Apple's ( and satellite content support servers ) there will be the normal folks on duty to catch any hiccups that leak through.
Apple's servers/networks are certain to get hammered for a while. Might as well have your normal folks, who got a good nights sleep, manning the ramparts.
Not that the servers won't be prepped for an onslaught but sometimes "stuff happens" and may need to fix something.
The secondary effect is hopefully get bit of a wave as folks wake up.
[ This is another one of the overlooked problems when they implied would launch worldwide on a single day. You'd get a bigger wave, longer aftershocks , and a substantially more complicated logistics. Doable, but why inflict that much pain on yourself on the first time out of the gate. ]
What "normal" sysadmin/netadmin do you know is at the office before 5:30am on a normal day? I would think midnight would be much better because you could update/role the pages and open the floodgates at a slow time to catch any problems. Starting the sell at 5:30am PST/8:30am EST will cause an immediate hit/load and leave little room to fix anything without pissing off a lot of people waiting to push the "buy" button.
Even if you started at midnight the time-zone effect would still stagger the load a bit.