Because I believe that Common Lisp is a better language than Clojure. I'd be happy to discuss specifically what I believe the most important differences are in case you're curious.

Clojurescript was chosen on the client side because its integration with Om was very nice and matched the idea of immutability in Clojure.

Now that Om is no longer well supported we have a problem. The client side needs to be rewritten, but none of us have the time to do so. We'll see what happens with the project.

This highlights another benefit of Common Lisp. It's very mature and I don't have to worry about the fameworks that I use becoming obsolete after a short period of time.

>Why not Clojure on the backend as well?

It is said that CL provides a nicer experience to the developer (like, for example, clearer error messages, a more interactive development experience, etc.)