Functional programming is harder to get your head around than imperative programming. F# is a more difficult language in many ways than C#. Most 'developers' don't understand functional programming concepts, and can't even write very good imperative code in C#. So what hope have they got of writing good functional code in F#?
And when you consider that everybody on the team needs to be able to understand, write, debug, fix, etc. the code in the language you choose, it means you need a very strong team -- not just a very strong person -- to be able to use F# as it's meant to be used. And there aren't many of those around.
Add into the mix the fact that there's 8 years of C#/VB code lying around which is unlikely to be migrated, and that it's easier to create libraries that look and feel like the BCL in C#/VB as it's less easy to leak stuff like tuples etc. through public interfaces, and I reckon that F# will struggle to gain anything more than usage by strong teams on new, internal projects.