Because it offers VMS niceties that UNIX clones still doesn't do, and stuff like AD, SMB, without manually going through configuration files stored somewhere, that differ across UNIX flavours.
Although I do conceed UNIX has won the server room and Windows Servers are mostly about AD, SMB, IIS, Sharepoint, Dynamics, SQL Server.
Naturally some of those can be outsourced into Azure services that Microsoft will gladly provide.
And to run windows only apps like some embedded toolchains. Although that gives a motivation for us to move on to gcc because windows is annoying to be used on CI/CD and gcc is good enough compared to that other toolchain