Teams is free to corporates
Corporates buy Office 365 for Word, Excel, PowerPoint and Exchange - must haves in most enterprise sized businesses. Upgrade to premium and you get Teams, SharePoint (which many want anyway) and Skype for business. All done with one signature from the CIO.
Forget about usability or features, corporate IT departments are looking at the dollars and ease of getting it approved. Why would any corporate even consider paying for Slack?