(Disclaimer: Teams is in my "red flag" list when evaluating a company - I hate it that much)
Teams is not popular because it does something that no other app does. It is popular (IMO) because it does everything (calendar, chat, videoconference, and wiki - all of it badly) and, if you're a Windows user, you're paying for it one way or another.
All that Microsoft had to do during the pandemic (which is when they unleashed Teams) was to approach a higher-up and pitch "why would you pay for Slack and Zoom when our product does the same? And since it's already included in your Office license you're already paying for it, so really, you're throwing money away". I know me and my friends complained about it, but so what? The company saved on licensing costs and IT people are always complaining anyway. And while the bundling of Teams got Microsoft in trouble in the EU [1] they still haven't paid any fines for it (I think) so it's hard to argue that they shouldn't have done that.
While Slack doesn’t do all of that natively. Everyone integrates with Slack. For instance if you get tagged in a comment in Google Docs you can reply to the comment in Slack. You can start a Zoom meeting from Slack and Google calendar (corporate) integrated with it.
Teams is not popular because it does something that no other app does. It is popular (IMO) because it does everything (calendar, chat, videoconference, and wiki - all of it badly) and, if you're a Windows user, you're paying for it one way or another.
All that Microsoft had to do during the pandemic (which is when they unleashed Teams) was to approach a higher-up and pitch "why would you pay for Slack and Zoom when our product does the same? And since it's already included in your Office license you're already paying for it, so really, you're throwing money away". I know me and my friends complained about it, but so what? The company saved on licensing costs and IT people are always complaining anyway. And while the bundling of Teams got Microsoft in trouble in the EU [1] they still haven't paid any fines for it (I think) so it's hard to argue that they shouldn't have done that.
</rant>
[1] https://apnews.com/article/microsoft-teams-eu-european-union...