My inference from the times I can remember him mentioning it: Basically, Steve likes dynamic typing. He feels that arbitrary language-enforced constraints just get in the way. Haskell is the most strongly statically typed language in existence, so obviously it's not his cup of tea.

If you read his most recent rant, you can see that although it's technically about visibility modifiers, it can read as a condemnation of compiler-enforced constraints in general.