Windows hosting is generally a requirement because of compatibility conflicts.
Clients commonly can have their websites designed on Microsoft’s Front Page Office program. Front Page uses a scripting language that does not work on UNIX and Unix-like systems so it leaves the client with only one option, Windows.
Windows hosting is also an option to clients when their databases and their developers work on Windows OS. If your documents and programs are commonly installed on a Windows platform it would make sense that if a server was incorporated then it too would run on a Windows OS. This would mean that there is a shared familiarity between the programs so that developers and general staff for the client company would be able to access, interpret and understand how to use the documents.
There is a reason why Windows OS has a lions share of the computer market for consumers, it is accessible and has become a tried and tested operating system in the process.