Commonmark isn't serious because it doesn't include several major quality of life improvements like [^1] for footnote/reference syntax. Pandoc should be the serious "common" markdown standard.
Part of the problem is that people want to say 'markdown' but John Gruber, pissing on the corpse of Aaron Schwarz, falsely claims total possession of everything that comes under that head, and that there must be no independent specification. This simple fact totally destroys the entire milieu. 80% of complaints about 'markdown' are due to his recalcitrance. Thus there can be no spec, no spec specifying extensions, etc. He does not want anything like html to be defined as he will lose ... I'm not sure what, since he is deeply evil, as this one case shows with total clarity. Industrial interests would be enough to get an html-like convention if it were otherwise.