Doing on premise is hard. Office networks have firewalls with not so great configurations. Mobile networks can be incredibly flaky and unless you have geographically distributed servers it's hard to guarantee latency and performance. While building Flock , we faced multiple issues with firewalls which took us a great deal of time to tackle to ensure that notifications work well ( We ditched TCP with TLS and came up with our own protocol similar to Facebook zero which relies on shared secured key for encryption). Cloud based solutions are much easier to set up and much more reliable compared to on premise solutions , which is another reason why Atlassian also created Hipchat cloud and Confluence cloud.

Disclaimer: 
I'm from Flock ( https://flock.com ) . It’s currently rated the #1 slack alternative on PC Mag(https://goo.gl/eGZqH8). We've also created a tool to help you quickly import all your team data from HipChat to Flock in case you're interested in trying us out. Please let me know if you have any queries !