Teams, as much as I may dislike it, seems to have more built-in features than Slack, including a files feature that supports editing MS Office documents in place, and integration with Outlook calendar and email and other Microsoft apps. I also think that Slack didn't have video conferencing until relatively recently?

As with the IBM model, I imagine it's simpler for companies to have a single source and a single support channel. It is possible to use Exchange sign on for non-MS systems and apps however.