This is the case for me. I simply don't have the time to invest in a technology that I keep hearing great things about, but from which I don't see many interesting things being built. I don't mean to knock on Haskell; I'd like to learn it, I just can't justify the time. I'm hoping to get there eventually, perhaps by pursuing Rust more or checking out Reason/OCaml, but I don't have many applications that could benefit from functional languages (at least not functional languages that are markedly slower than the languages I'm building things with today).