If Apple were just targeting the readers of MacRumors, and others with a big wad of cash in a jar with an Apple logo on their mantlepiece then that might not be a problem.
If Apple actually want to continue to expand their market, however, they need to reach a wider audience. One thing they've got good at in the last 10 years is turning their announcements into international news stories, and in that case launching too much in one go will dilute the coverage.
If Apple launch a phonebook full of products then MacRumors will happily cover the lot, but non-Apple-centric publications will have a limit on how many "column-inches" (or the electronic equivalent) they'll want to devote to Apple in one go. If Apple were to launch iPhone 5, iPad Mini, new iPod range, new iMac then a lot of non-Apple sites will just pick one to headline and discuss in detail and mention the others in passing. Give it a few weeks between each launch and Apple can score a headline for each one. Also, if there are any 'controversies' (e.g. "New iPhone drops telephone function - 'nobody uses iPhones to make calls any more, and the carriers are a bag of hurt' says posthumous Jobs memo") then although there ain't no such thing as bad publicity, the iMac isn't going to get much coverage.
Then there's who to invite to the launches (which will always be over-subscribed): you don't want to bore the Music press and Youth magazines with new iMacs before/after you show then new iPods and music services, the 'mainstream' press will be more interested in the iPhone/iPads. Having separate events lets Apple target them.
Finally, there is the logistics: generally, when Apple launch things they like to say 'available now' or at least 'available in 2 week's time'. C.f. other companies who risk the Osbourne Effect by having a new product announced when the last product has barely made it into the shops (*cough* ASUS *cough*). That takes a lot of logistics and even a big company like Apple can only do so much in one go.
That's not to say Apple won't launch multiple products - they do seem to have got themselves into a bit of a hole whereby everything other than the MacBooks are overdue for an update. Personally, I wouldn't be surprised to see minimally changed iMacs with Ivy Bridge and USB3 quietly snuck out in a press release.
