My impression is that the high salary is because Clojure it is primarily used for small-ish, fairly senior, highly technical teams writing high impact standalone services e.g. it's my understanding that a Walmart's route planning teams writes their service in Clojure.

This actually meshes well with what you'd expect from a lisp being used well. Lisp's power and its downfall is that it lets you express abstraction at whatever skill level you're at with no built-in mechanisms to keep style consistent across large teams. This works really well for small teams of high powered developers.