I wrote a book on FreeBSD 20 years ago. And today I make money from Windows Desktop, Windows Server, Ubuntu and FreeBSD.
You have to keep in mind that Windows Server costs money so nobody is going to buy it just to bang around on and have fun with. They buy it because they have an app that lists it as a requirement.
You take for example Clearview software, they have products that require Windows Server + IIS + SQL Server. (SQL Express works fine) However, IIS, as you know is a very poor web server for a lot of reasons.
But, Microsoft supports it like the dickens. If a developer at Clearview runs into a problem they pick up the phone and call Microsoft and Microsoft goes out of their way to fix the problem maybe even doing significant programming work for the Clearview developer. That is why Clearview doesn't support Oracle or Apache on Windows or any of that; just Microsoft products.
Even though you hand out the code for a free Windows web server, since IIS ships at no cost with Windows Server already, it is zero-cost vs zero-cost and the decision will then be made on who provides more support. And that's Microsoft.
There is a huge need for OSS on Windows. But not for products that go head to head against Microsoft products that are zero-cost.