Markdown is popular because it is simple. I have never had to teach anyone Markdown. They look at the file and immediately see what's going on and can copy the style.
Occasionally someone may have to look up something like table syntax, but no one in my career or personal life has ever asked me to explain Markdown to them, which is rare for a technology that is so widely used.
This has not been my experience with other markup languages.
Bonus: It starts simple but can be used to do whole books if needed.
If you introduce something with advanced functionality onto a team, soon enough someone's going to use pieces that others don't fully understand later.
Now everyone touching the document has to understand includes and document metadata and whatever else someone added. Suddenly you have includes and cross-references and what used to be a document where you could read the raw non-rendered form linearly top to bottom is a confusing mess unless you render it. Markdown is almost always just as legible raw as rendered.