[lilyball]

Not all markdown implementations are CommonMark

[yencabulator]

There's not much reason to be anything else than CommonMark + extensions.

[lilyball]

For new implementations, sure. But it's harder to change existing implementations (anything not already CommonMark-compatible will introduce unexpected changes to existing content if you switch to CommonMark), and especially for anything that's not being actively developed it's unlikely to ever change.