What learnt in school is insufficient to meet with all real world challenges. To claim as a developer, you need to understand the internals, that is the ability to poke and trace to the root of the problem deep underneath.

Debugging in FP feels different to most people, where our thoughts is tuned to the procedural model with observing states and form our hypothesis from there.

Another disadvantage with niche programming language is not enough Stack Overflow help there. That alone will drive most people away.

I don't know how it is elsewhere, but I saw many (most?) students struggle with those classes. These are people who have learned to successfully program, but they struggled quite significantly with functional languages (Haskell in this case). Even when they pass, many of them probably wouldn't want to pick up a functional language afterwards.

But Scheme and Common Lisp (and its dialects) are drastically easier to learn than Haskell.

Furthermore, I think it also has to do with the order in which these courses are taught. When I did a small introduction to FP at my university, a lot of people who had no prior experience in programming picked up Haskell much more easily than those who already had a background in imperative/OO programming.

(Though that strengthens the point - FP is often quite a leap for programmers who have used imperative languages for a long time.)

People have limited time in their lives. If you want to recruit volunteers to your open source project it has to be in a language they either already know or are interested in learning. It may not be “hard” to learn Clojure, but the potential contributor may consider the time and mental energy spent fumbling around frustrated with all the off-putting sigils and different brackets to be better used learning a language with brighter prospects.

Many cs courses teach ML style functional programming, not Lisp or Clojure