How Microsoft Crushed Slack

Microsoft didn't crush Slack, far from it.

Microsoft Teams is a steaming pile of shit. It can never win on technical merit against competent (or even semi-competent) competition like Slack, Mattermost, etc.

The only reason Teams is seeing supposedly good adoption numbers is because it's bundled with Office 365 at no extra charge and in a Microsoft environment the thing auto-starts with the OS and logs you into your domain account, thus counting as an "active" user regardless of whether you willingly use it or never touch it. It's also the official replacement for Skype for Business (formerly Lync) so companies already deep into the Microsoft stack (parts of which are good, but the majority is crap) are following the "safe" path and upgrading to it instead of exploring alternatives.

Teams is not really "winning" marketshare in the sense that they're just capturing the majority of Skype For Business / Lync users, but no modern company evaluating these tools on technical merit will pick Teams over Slack.