Bit of a far-flung theory, but I think it might have been a deliberate ploy to cover their tracks and keep their identity a mystery. If it was someone closely connected to Unix development, but they didn't want people to connect the dots and discover they started Bitcoin, what better way to do it than to make the first version Windows-only? Secure in the knowledge that if it was successful, it wouldn't take long for Bitcoin to migrate back to Unix, the spiritual home of open source. Never underestimate Satoshi's brilliance and forward-thinking.
I don't think it's far flung at all and you're right, this to me is a perfect example of forward-thinking... that actually you might realise seems to remain with the bulk of Bitcoin core developers.
Satoshi were a group of highly intelligent people - near flawless communicational language, extremely sound coding. From the beginning, they were already keen to safeguard their identities and part of the strategy would have to include deliberate inconsistencies in character, to complicate any potential profiling they were sure to attract. I think using an OS more common for programmers is part of that obfuscation, and would have helped narrow down any such attempt.
It could also be practicality. If you wanted adoption beyond the confines of cypherpunkery you want Bob and Alice's PCs to be able to run the early clients.
Ah, good, so it's not just me then. So if we are looking at prominent Unix developers, is
this theory plausible? It would certainly explain Satoshi's absence, but obviously not in nicest of circumstances.
It just seems to fit somehow. Think about it this way, if you had been a primary contributor to something as innovative as Unix, you had now retired from working, you had experience with ciphers and cryptography, plus you just so happened to be a goddamn visionary genius, why would you
not take a shot at trying to fix money by applying the same open-source principles?