Because PC's were maybe only 60% of the actual software development market and there were a lot of us hackers whose "parochial" needs involved getting stuff to work across both platforms. Were you actually there?
I mean, Notepad is a silly thing, sure. But the crazy spec incompatibilities between MSVC and the C9x and C++03 standards were a huge source of grief for me personally. And the mess caused by Microsoft's "Java" implementation was straight up evil.
From Microsoft's point of view, when they held >90% of the PC market, what would be the point of even thinking about interop?