I assume that F# developers bring better business outcomes.
I am both a C# and F# developer, and in my experience functional code (F#, OCaml etc) tends to be 5 times less buggy than an imperative equivalent. F# compiler does an impressive job of catching errors at compile time: this is the primary reason for better outcomes. The second significant contributing factor: functional code tends to have less side effects, and thus developers can reason about such code more realistically.
A third contributing factor is lazy computations. Which leads to (surprise, surprise) better overall performance at runtime.
Less bugs + Better performance = Higher salary
Though I don't say F# is perfect. It has its quirks and there is a lot of room for future improvements.