From the outside, Teams (and Zooms) growth was accelerated because video conferencing became way more important. People didn’t need video conferencing pre-COVID because that would have just been a meeting. When they suddenly did, Slack wasn’t that good. My pet peeve continues to be that on Slack I cannot see someone’s screen on iOS - that one annoyance had us move to Zoom then teams.
An argument could be made that Slack was ultimately unable to compete in video by virtue of not having an army of high paid engineers to throw at the problem like MS does, but the features that Slack chose to focus on (like the WYSIWG editor) doesn’t say that to me