I think it is a little harder to set up (compare to Ruby and Python, which are preinstalled on Macs), plus throw in the functional concepts,Lisp syntax, and Java errors. Together it makes for a bunch of small initial pain points that I am certain causes some people to drop out, which is frustrating because Clojure is awesome. DrRacket might be a good comparison for a more similar language with an easier initial ramp up.