Haskell compiles so slow because it needs way more optimizations then Ocaml before it performs good. The primary reason for that is its lazy evaluation semantics. Prior to GHC many researchers believed it to be impossible to execute lazy languages with comparable speed than imperative ones. Haskell and GHC were primarily developed as research tools. I.e. they valued the efficient exploration of new ideas in programming languages and implementations higher than the resulting language and implementation, at least in the beginning. And I would say Haskell was extremely successful considering those priorities.

That does not mean, that the implementation hasn't been moving towards industry-readiness for a long time.