What Microsoft did was, they said "Hey we have a monopoly on office software. We should have a monopoly on chat too". And they started giving away Teams for "free" i.e. all companies who were paying for an Office subscription were suddenly paying for Teams too with no way to opt out. How is someone going to compete, when most of their potential customers are already forcibly subscribed to a "good enough" competing service.
It was especially egregious here because Teams was such an inferior product so it probably won in no small part due to this tactic.