It's also great for writing documentation in a plaintext IDE, which I think is what _really_ drove a lot of the adoption.
That GitHub used it as a "native" format everywhere from the beginning (as far as I remember), probably helped Markdown become at least as popular (or maybe even more) as GitHub itself.
Then everyone and their mother started doing static blogs, and since people already wrote their READMEs and issue comments with Markdown, I guess it was natural to want to write your blogposts with Markdown too, just like Gruber.
My company is on Teams and I regularly use Markdown in my messages, though I still struggle to remember that I have to use underscores not asterisks for italics.
As for Teams, it looks like it’s much closer to Markdown (uses the same idiosyncratic/stupid link syntax), but still significantly incompatible even if they call it that. And my guess (as a non-user) is that it’s just an input method immediately converted to HTML or similar, not retained as text. So in that way it’s not Markdown either.
It helps that Jekyll, one such static blog, was also pushed by GitHub back in the day.