We discussed this on another thread recently. Aside from Spinal Tap-esque marketing which is almost certainly the main reason, I suspect that the other driving force is to allow them to align their desktop product with Windows Phone releases.
It’s a cute idea but I don’t buy the “if Windows version starts with 9” story. a) I’d be stunned if anyone ever detected versions in that way, the internal version of Windows 95 isn’t “95”, it’s “4.00.950” (W98 is 4.10.xx) so you’d say something like “if majorVersion==4”, and
b) even if they did, I can’t for a second imagine that it’s would be such a big deal in 2014 as to influence Microsoft’s product naming.
But yeah. It’s Windows 10 and not 9 because it’s 1 louder.