Marketing
A simultaneous worldwide launch is not a cheap thing to do. Originally Microsoft planned to do exactly that, but gradually scaled down its plans. Generally a company will leapfrog from one major territory to the next, typically first Japan, then the US, then Europe, then wherever's left, with cashflow from one territory being used to finance the move on to the next.
Microsoft instead launched in Japan, the US and Europe relatively closely.
It's highly regrettable that Microsoft haven't included Australia in this group. (I wept openly on hearing it.) Normally Australia is considered part of Europe. We're a PAL territory, and we speak English, so we often get Britain's leftovers. And we should be glad, because otherwise we'd get squat. Unfortunately for this one we're being classed as Asia Pacific, and have have to wait until now. Microsoft cites limited console volumes as a cause, but that isn't really very satisfactory. Australia once again is treated as the minor market we really are but resent being. It's hard to look at the vast markets of the US being given so many consoles, and not wonder why just a few crates of the hardware can't make it over here.
Naturally the US launch has been plagued with shortages, as has the European launch, but it's still hard to regret not being even short-changed on this one.
Hell, just ship us some of the ones bound for Japan. We predicted long ago that they won't be buying them anyway. Japan has not supported the Xbox at all, and yet all the love goes to Japan. Hard to take. Especially since we were right, and stock languages on Japanese shelves, where cute schoolgirls walk past and giggle at it.
In any case, by adopting a global (except Australia) launch strategy, Microsoft have anted up in a big way. It's easy to dismiss this by saying they've got trillions of dollars, so no big deal, but even for them this represents a big investment. More importantly it represents a big commitment (though not to Australia. Apparently we smell). They have been pushing the Xbox 360 hard from the beginning (except in Australia).
Still, Microsoft's decision to go world-wide (except Australia) is both brave and brilliant. By launching worldwide (except Australia) Microsoft ensure that they can capture the lucrative pre-Christmas buying market all over the world (except in Australia). The significance of this cannot be over-stated. Microsoft needs to get as many consoles in homes as they can in the pre-Christmas splurge, so they can start with a significant install base when the PS3 launches.