It is not. You should check it out some time. For purposes of this post I will pretend haskell and ghc are one in the same (ghc is the most popular haskell compiler). Haskell is built around a very simple, straightforward core language (shockingly enough called "core"). There is massive resistance to adding anything to haskell that can not be expressed in core and would require changing it. Haskell values simple, powerful abstractions that can be widely re-used. Haskell has not taken over the world because it doesn't want to. Python has not taken over the world either, I doubt many people would claim it is because python is too hard or too complex. Taking over the world does not appear to have any correlation with being a good language.