When you don't know a lot learning something different is actually easier than when you're already proficient in C#. You expect an intern to take x ammount of time before he can be productive, so if he spends it on learning how to do it with F# or C# it won't change the x much. But when you have a senior who knows how to do something with C# he will not want to invest ammount similar to x, even a 1/2 of the time, because he can get it done with what he knows.

Like I said to OP F# doesn't have that "hard sell", it's just a bunch of incremental improvements that end up being a big deal together, but each one on it's own is unimpressive.