If you'd used Teams, you'd know why. It is hilariously bad, gratuitously slow, ludicrously weird. I have periodically had to use Microsoft's team chat tools over the years, dating back to Yammer or perhaps even further, and for whatever reason it seems they just do not get chat.
Better question: why not just switch to Mattermost?
I use both all day, every day. I don't find Teams to be any worse, slower, or weirder than Slack.
Teams also does a lot that Slack can't do. As one small examples, it seems amazing that I can't add tabs to a Slack channel like I can in Teams.
You might not even be able to, Teams API is quite badly convoluted and limited. You simply won't be able to f.e. recreate Slackbot experience in group conversations(chats).