Because it’s great, and people that think otherwise are dead to me.
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It's the one programming language that changed how I think about programming.
I'm only talking about the version before type providers. Then it got messy.
Before that, we could (and I did) recompile fsi.exe to do some custom prompt manipulation. It was a slog, but it worked, but then Microsoft faded from my life. Still, that early version (I believe 2.0) F# is just magnificent.
My current work needs nothing the .NET environment provides that I can't use python's standard libraries to get done, or bash and C if I need to.
But I'm lucky to no longer be in a corporate environment, so I don't need to consume commercial services, which was much easier using WCF within .NET. Back in my previous life, constructing n-tiered services on top of SqlServer using WCF was slick, indeed.
To any who are interested in how to construct such n-tiered applications simply but securely and precisely, I highly suggest Juval Lowy's IDesign system. He had three specific videos that I watched three or four times each until I understood his distillation of his vast expertise. Of course, Mr. Lowy is one of the co-designers of WCF, which was an excellent bit of tech.