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.
I believe the part about "extremely sound coding" is wrong.I have heard some comments that Bitcoin's early code was full of bugs and was basically spaghetti code and it started to improve when the first contributors came in.
This made some followers think that Satoshi was a good cryptographer and mathematician but he was not a good programmer.