I think it has to do with active choice. The first programming language that people are introduced to are unlikely to be the "best" for them. The first programming languages are disproportionately java, c#, PHP these days. By extension the talented people mostly move on as they learn new and interesting ideas. This leaves the pool of developers poisoned by the mediocre or bad people who spend their entire careers being expert beginners in one domain. People come to the more exotic languages, like Haskell, as their second, third or fourth language. By default the average developer in that pool will just be much much stronger.

Just because the average developer in a language is bad it doesn't mean that the language itself is bad.