Can anyone please explain why Slack is so popular?

Isn't it just another IM chat? I've tried it a couple of times and found nothing special about it.

Also, in all companies I worked at they either had Lync or Skype (because Enterprise, Windows, Office suite etc.).

Even if company decides to try some fancy IM, isn't it better to switch to Atlassian HipChat as it gives nice integration with JIRA, Confluence, Bamboo, BitBucket (assuming company uses Atlassian toolchain)?