[Canada]

It's a scalability problem.

If you have many web servers behind a reverse proxy that takes care of TLS it's often pointless to waste processing power on encryption in between.

[anthony_franco]

Yep. And on top of that a reverse proxy is unable to inspect the requests to route them smartly (based on path, cookies, etc.).

[Canada]

Well, the reverse proxy would have made the backend request itself, so it's free to inspect or modify anything.