People use slack because they liked it and convinced their managers.

MSFT has direct access to every single CTO/CIO in the world. They have a sales pipe into most of them along with a bunch of other products. 'Teams' is on paper, the Slack equivalent that rolls in along with the other products and will likely be preferred by Managers.

I believe that Discord has 1) more resonance with XBox than Teams, and 2) it gives MSFT access to a true, consumer, content oriented setting. Like FB, TikTok etc. which is a cultural access point they don't have.