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For a few years I got to watch a lot of novices and young programmers come to grips with various topics/programs together while I was helping out with a CS summer camp for prospective students at university. One year I was involved we used Pascal, another year we used JavaScript, but for a number of years, we used... Prolog!

This was done partly because prospective CS students often already knew Pascal, JS, or both -- putting the curriculum in a lesser-known language provided some extra incentive for those looking to stretch themselves.

The other thing, though, is that it seemed to put our smart but novice students on more equal footing with our students who already knew how to program. That is, though Prolog was arguably "harder", our novices seemed to keep up.

I suspect that Prolog isn't really harder, it just requires some thinking on an orthogonal axis to more common languages. Most of us don't get much practice on that axis, so writing programs in it is difficult.

The same thing may well be true of Haskell and Lisp.