Anecdotally from people I know from MS is that adhoc slack usage was popular up until lockdown, at which point more resources was poured into Teams due to everything going remote and internal slacks were frowned upon.

I see a parallel here where a competitor's product is taking over and MS leaders see it becoming an existential problem and are putting their foot down and pushing internal users to the company's products.

Also if half the company don't even want to use something made by the other half it's a bad look lol