Skype was a P2P phone calling program over TCP/IP.
I.e., a neat trick that might have killed traditional telecoms.
Then Microsoft bought Skype and killed it instead.
Even so, Skype was a household name for video chat for years (despite it's sometime shoddy quality) until FaceTime, Gchat and others implemented video towards 2010.
What's Slack done that's different from other chat platforms available today? Stuff like Jira integration is of interest only to a small portion of its target market, and it's that exact market that will complain about the pricing, that it's non-federated, heavy on resources, intrusive in its tracking, etc.
Btw, although I've seen little difference between HipChat and Slack in my day-to-day, the switch from Skype to HipChat was a profound improvement. Messages sent on Skype could and would frequently vanish into the ether, and/or arrive in the conversation thread way out of order. Imagine troubleshooting production issues under those conditions!...