I am not the expert on this subject (Michael Kaplan might be a bit closer), but I can speculate on the reasons for this. This is all conjecture, so who knows how accurate it is. (Actually, most of what I write is conjecture; I just don't bother calling it out each time it happens.)
Let's look at it this way: You have the time and money to translate the American English version of Windows into 20 other languages. Do you spend one of those slots to translate it into a language that is mutually intelligible with American English? If you have to choose between a language which is already mutually intelligible with one you already covered or a language without which an entire audience would be unable to use Windows, which are you going to pick? In concrete terms: Which is more important, that people who prefer British English don't have to suffer through the indignity of reading American English, or that people in Thailand be able to use Windows at all?
Okay, maybe if you're one of those indignitized (indignitised?) British English speakers, then you don't mind that an entire country of over 60 million people lose access to Windows so you can see your words spelled correctly. (But then again, you'd still have to take a back seat to Indian English, since they outnumber you by a huge margin.)
Mind you, the English language isn't the only one with this problem. There are dozens of variations on the Spanish language, but Windows chooses just one of them as its "Spanish language edition" and people who prefer some other variation will just have to suffer through a translation into a nearby (but not perfect) dialect.
My premise destroyed: Then again, Windows is available in both European Portuguese and its upstart offspring Brazilian Portuguese, which as far as I'm aware are for the most part mutually inteliligible. So who knows what the criteria are.
Bonus: Aha, it looks like Michael blogged about this after all.