That’s because Gruber shut down the effort to standardize markdown. He declined to participate and then demanded that “Standard Markdown” be shut down when it went public. This is what yielded CommonMark.

https://blog.codinghorror.com/standard-markdown-is-now-commo...

So we are left in the mode where markdown remains needlessly fractured. Different flavors that render slightly different and everyone gets a little annoyed but they deal with it ands occasionally bemoan the fact that there’s no standardization.