Seriously, the average programmer trying to learn Haskell starts with wanting to print "Hello, world", eventually winds up at a tutorial about monads, then retires with their head spinning. Haskell remains on the, "I should learn that some day" bucket list and remains unlearned.

This is not to say that you don't have plenty who don't learn them. But now we have another problem. One of the biggest reasons to use a language is available libraries. Because of the initial barriers to entry for these more sophisticated languages, there is a smaller pool of people writing useful libraries. Which means in the real world that when you want to get something done, you'll be more likely to find what you need pre-written if you use a more mainstream language.

Just to get a sense, in the (admittedly highly flawed) TIOBE index, the top language with a strong inference system is Scala, and the next is F#, then Haskell, and nothing else is in the top 50. The sum of popularities for these three would tie with Groovy at #18.

I have never written anything more than a toy program in any of these languages. I doubt I ever will.