unknown slack is a widely adopted chat system, but it becomes expensive easily (integrations and history for instance). Discord has none of these issues, offers chat, voice, screen sharing out of the box, for free.
To be honest, I have been a nomad when it comes to finding an enterprise-grade chat solution, having tried Slack, HipChat, Mattermost, Skype and many outliers. But only Discord, so far, has proven to be stable, offer all necessary features and demand a low financial footprint.
This is my opinion, the final decision wasn't made by me but it might have been affected by my enthusiasm ?