Mostly because of user experience. F# as .NET language has entire ecosystem of high quality libraries to pick from, good IDE support (not as good as Java/C#, but definitely better than Haskell) and smaller learning curve. It's also way more robust than Rust - meaning that you can make a working project with decent performance much quicker. I say that as both F# and Rust developer. IMO the language that covers similar area and may be more tempting to learn is Scala. But if you already know how to utilize .NET platform, then reusing that knowledge in F# is just easier.

Basically what Horusiath said.

I'm a big fan of C# and the .Net platform as well. Being able to mix C# code in a project is compelling. If C# had a strong native SSH(I'm aware of netssh, but something a bit more official/active) implementation, and a WinRM/Remoting implementation that wasn't hidden inside the Powershell project, I think there would be a .Net OSS tools explosion..