Short version, IIRC: Microsoft made MS-DOS, but their application software offerings for MS-DOS (Word, Works) were not compelling enough; other vendors had that market (WordPerfect, Lotus Symphony) pretty much sewn up. Then came Microsoft Windows. Other companies did not want to spend huge amounts of resources porting their applications to what was essentially a niche Microsoft add-on on top of MS-DOS. Microsoft, on the other hand, had every reason to bet big on their own product, and they invested massively in porting their own software to Windows. Then, when Windows became popular, all other application software vendors were way behind in producing reasonable graphical Windows versions of their software, except Microsoft, who gained a massive user base of the Windows versions of their programs, since they were much better in the Windows environment than everything else. Microsoft used this advantage to stay on top of the PC market for years, and arguably still do.