I can think of a few reasons: same language between front and backend, extensive standard library, repl driven development, simplicity, brevity, spec.
They've done a wonderful job with Phoenix and Elixir. I think this feature liveview will be develop as good as presence feature and eventually will be good enough.
anyway i still don't see why i would need it with Clojure if i can have single codebase and language for backend and frontend and render whatever i want at either -end.