I don't know about the overall reason, but in my personal experience Slack just worked way better. My team used Hipchat for a while a few years ago. We found it

I don't know about the overall reason, but in my personal experience Slack just worked way better. My team used Hipchat for a while a few years ago. We found it to be very buggy. I would frequently send messages, but the recipient would never get any indication I'd sent it. The same thing happened for my incoming messages. Technically the message would be there if I looked though all my chats and checked for messages I hadn't seen yet, but there were no notifications or indications they were new. Even when the worked, Hipchat notifications weren't configurable a the granularity I wanted. Slack also is prettier and feels more cohesive to me, but that's a secondary concern.