M$ was too late to open source it and too late to embrace Linux. On desktop side .NET was only for windows back then and xamarin immature. On server side in java land you had framework war so you had many choices and libraries to choose from - companies didn't want to fully embrace M$ ecosystem (windows sql server, windows deployment etc). Then Android came and adopted Java and M$ lost mobile ecosystem war.

I can see similar way why Swift eventually can loose market share - too late to open source and too late to embrace cross-platform.