The big sells for Hangouts and Teams is that they have video. Slack doesn't so you have to find a video conference provider as well like Zoom. Also Teams integrates more easily with the existing Microsoft stack (office, outlook, etc.) that a ton of non-tech companies are on.

I think a lot of tech companies struggle to break into the non-tech world. Slack might be ubiquitous for SWEs but my SO who works in traffic engineering will probably never use it (they use Teams).