The value of Windows is in the Win32 API, due to the existing investment into software. If you have existing code base, you want to keep it running with minimal ongoing investments.
For a case study of interest for newer APIs, see also Windows RT.
Windows RT failed, because it had no applications, except for MS Office.
So Win32 was there, ISVs just were not allowed to recompile their apps for the ARM target. They were expected to port them to Modern API. Which of course, they didn't.
For a case study of interest for newer APIs, see also Windows RT.