Keeps support for poorly coded programs working. In the old days, a quick and hacky way to determine which Windows version the system was on was to have the program check the OS name. If the name started with the characters "Windows 9" you knew it was either Win95 or Win98 and ran in one mode, but if it was something else it ran in the other mode. If the new OS was named Windows 9, then certain old programs would break when run on it. Yes, the people who would have coded that way are idiots, and sure, the number of people running those programs may be in the single digits, but Microsoft has been pretty serious about maintaining backwards compatibility, even if that means ever more cruft and jank.

The other reason is marketing. "See? It's not anything like that awful Windows 8! We skipped all the way to 10 to demonstrate how different it is! Please come back!"