Whereas most of the Unix/POSIX/etc world uses UTF-8 for text representation, Windows uses UTF-16LE.
Why is that? There are multiple folks who say the Windows APIs were written before UTF-8 (and even Unicode as we know it) existed (1, 2, 3), so UTF-16 (or even earlier, UCS-2) was the best they had, and that converting the existing APIs to UTF-8 would be a ridiculous amount of work.
But are there any official sources for these 2 claims? The official MSDN page for Unicode makes it seem like UTF-16 may even be desirable (though I don't myself agree):
These functions use UTF-16 (wide character) encoding, which is the most common encoding of Unicode and the one used for native Unicode encoding on Windows operating systems.
Is there any official note (or an engineer who worked on the project) explaining the reasoning behind choosing UTF-16 and why Windows would/would not switch to UTF-8?
Disclaimer: I work for Microsoft.
Raymond Chen actually has an "official" answer—or at least an answer from a Microsoft source (emphasis added):
Windows adopted Unicode before most other operating systems.[citation needed] As a result, Windows’s solutions to many problems differ from solutions adopted by those who waited for the dust to settle.¹ The most notable example of this is that Windows used UCS-2 as the Unicode encoding. This was the encoding recommended by the Unicode Consortium because Unicode 1.0 supported only 65536 characters.² The Unicode Consortium changed their minds five years later, but by then it was far too late for Windows, which had already shipped Win32s, Windows NT 3.1, Windows NT 3.5, Windows NT 3.51, and Windows 95, all of which used UCS-2.³
— The sad history of Unicode printf-style format specifiers in Visual C++
In other words, Remy Lebeau and AmigoJack were both right—Windows adopted Unicode before UTF-8 was recommended (or even existed?); at the time, UCS-2 was the standard, so that's what Windows chose.
By the time we developed the more efficient (and now more-common) UTF-8 standard, Windows had already shipped several versions, and it would be immensely impractical (if not impossible) to change.
Thanks to everyone who provided answers to this question! Since I was looking for an official source, I'm marking this as the answer (although I'm marking it as community wiki, since it is an amalgamation).