The difference is Clojure's persistent data structures. You don't add an element to an array, you create a new copy of the array with the element attached, and the old array is accessible.

Internally this is done with structural sharing, so it's way faster than copy on write, but it's still potentially allocating a half dozen objects for what would usually be a trivial operation that does not involve allocation.

On the plus side, Clojure in theory has some pretty nice guarantees that older objects can't point to newer objects. From memory Haskell takes advantage of these guarantees to accelerate its version of eden. There may be a way to get a fast GC that's not nearly as complex as the full JVM GC.