1. Apparently simple enough to implement to get the basics right (not that I could do it)
2. But there is plenty of room for accidental edge cases in the implementation
3. Many then implement it with or without edge cases
4. But it’s not enough because a lot of the people who use it are programmers so they naturally want to extend it to do just a little more
5. Now the standardization effort (Commonmark) has to take all of those edge cases into account plus have to figure out how to deal with extensions (especially for popular ones)
6. Then you get a heroically (in terms of effort and grit) wordy standard
All for the initial idea which is so simple that it seems redundant: look as good plain as rendered. (Then what’s the point of rendering it, and in turn what’s the point of a formal syntax instead of the (still existing) ad hoc email and whatnot conventions?)
Right now it’s fantastic for chat applications (here you definitely want rendering and not the visible markup). It’s been pretty good for things like forum comments as well. With the exception of block quotes which are a pain to fiddle with (the only prefix syntax if you don’t include the indentation-for-code since that is de facto optional now that so many applications support code fences… all because that’s how email does it).