To be clear, I agree that it could be a reason, along with a multitude of others. I think that discerning which is the most substantial reason (if any such exist) is hard if not impossible.
I think the pattern that emerges is that the popular languages I listed are multi-discipline, they can be adapted to whichever paradigm you prefer, even if it's a little cumbersome, and over time they adopt the key features of other languages, while retaining their existing benefits. In other words... you can get the best features of Haskell in Python, but you can't get the best features of Python in Haskell?
Haskell's best features relative to more mainstream languages are HM-style type inference, higher-kinded types, and typeclasses - none of which are possible in a language without real static types.