The CommonMark spec even has an example test case! The excuses for poor implementations are pretty thin.

Not all markdown implementations are CommonMark

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

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.