Nobody bats an eye when you use Python for logic, sql for querying your database and regex for searching text. Unlike the front end, there are ample opportunities for "uniting" the backend in one language, yet nobody really seems to bother.

I speculate that it's because these languages are better at certain things, just like having a non-executable language for data, helps keep the data, data only (cue angry lispers), and facilitates easier writing of prose in a mark-up language than a real programming language could facilitate. Just look at markdown.

More over, we invented JSON, rather than just sending JS files between browser and server.