I didn't assert that more people know OpenCL than functional programming, I said far more people know C than functional programming. Anyone who knows C can write (bad) OpenCL after a trivial tutorial, so the initial barrier to entry is low. Writing decently optimized OpenCL is, as I said, moderately difficult and requires some domain specific knowledge, and it scales up from there if you want to get close to optimal.
Unequivocally, C. Futhark is clearly meant for reasonably high performance computing and I don't know of a single person in that domain that isn't a competent C programmer even if they spend a lot of time writing code in other languages (C++ mostly, also Fortran, Julia, Python with Cython/Numba, etc.). Functional programming, on the other hand, is something many (most, I'd contend) people in this domain consciously stay away from, myself included.
