A product that focuses on something is going to be better than something that's treated as "just a feature" in something else. And for "chat" they already had (not great) messenger offerings, but didn't make the jump to group chat. And Slack had benefits from being able to position itself as a cool new alternative, and from not relying on already-existing central infrastructure: it could establish itself at the core of new companies that e.g. didn't use exchange and it could easier be sneaked into large orgs as a grassroots thing - just signing up your team for Slack was probably easier in many places than getting IT to give you an extra feature to the central infra.

And now that Microsoft is paying attention, it's doing very well at fighting Slack, despite Teams' weaknesses. (They also tried to buy Slack first) But it took Slack to show them that a new group chat product was needed and valuable.