HipChat is known for having a problematic low-level protocol (based on a fork of XMPP), it's featureset is much more limited than Slack's, and it's implemented in a different programming language.

It'd be a lot less work to fork Slack and make it work on-premise than do something with HipChat. And that's saying a lot: forking Slack to make it run well on-premise would likely be an enormous project.