I imagine that you either had a prior knowledge of Java or that you had to learn it to be a better Clojure programmer?
The stereotype is that Clojure programmers are grumpy bearded Java programmers who manage to sneak in Clojure into existing Java code bases to avoid flagellation by their corporate overlords. This isn't universally the case but does seem pretty common judging by the Clojure conferences and what not.
Is ClojureScript actually used for front-end development or are they just trying to push it? Maybe the huge size of Java's ecosystem is a barrier for aspiring Clojure programmers.
I don't know anything about ClojureScript, but I certainly think that Clojure would be unusable for the target audience without Java's ecosystem.