Excel and Word shipped first on the Mac and they were a couple of the best apps on the Mac at the time. Windows was born of Gates' frustration with Apple's low marketshare and refusal to license the OS externally. He wanted a more widely available platform on which to sell his warez.
It was never about building a great OS, it is about providing a compatibility layer for Word and Excel.
The original Word and Excel apps on the Mac really were great apps in a lot of ways (although they had an annoying habit of bypassing the Mac Toolbox to do things which resulted in compatibility problems for lots of people).