I could also find:
Besides from that, I would consider that:
* This is pretty basic operation on sets
* Not everyone comprehensively knows set theory
* Sometimes people imagine they could do X, but don't, if there's no readily available solution
* By offering this op, people can discover/remember that they can possibly want such a thing
The last point being the most important: people won't use (or think of using) things they aren't familiar with. So Clojure can help teaching/reminding these classic concepts.
Otherwise, the lack of popularity can also be considered sort of a self-fulfilling prophecy?