Its the integrations stoopid
Slacks USP is the integrations. It basically makes the hipster devs never have to leave it. It essentially becomes their comms and notification dashboard. Once you have JIRA, Jive, SNow, Confluence, Webex, Quip and half a dozen others all configured they are in happy land.
It becomes very sticky - precisely because its the glue holding the whole SDLC lifecycle apps together.