It is stil not a specification like restructuredText[1]. Also wikiMarkup (which really started this markdown) is different from GitHub markdown, which is different from other markdown editors. Also many sites use their own markdown versions.
If you are in restructuredText world there is one specification and all implementation adhere to it, be it pandoc, sphinx, pelican, nikola. The beauty of it is that it has extension mechanisms which provides enough room for each tool to develop it. But markup can be parsed by any tool.
Because its inherently appealing, close to what you wanted intuitively, and if you're only dealing with a single implementation of it, works fairly well.
You don't really get bit by its lack of a standard and extensibility until after you've bought in.
It's essentially designed by the opposite of a committee -- rather than including everything but the kitchen-sink, it contains support for almost no usecases except the one. Which is very appealing, when you only have the one usecase.
Well rst is better than markdown from day one. The only reason it became famous is thanks to wikimarkup.
So markdown needs to thank the popularity of Wikipedia for its success, as rst did not have any application like Wikipedia. But still rst is used widely enough with its killer Sphinx, readthedocs and now its kind of de-facto documentation writing markup in Python and many open source software world.
Because you can teach someone markdown in five minutes. And even if they don't know all the ins and outs, the basics are pretty foolproof (paragraphs, headings, bold and italic).
If you are in restructuredText world there is one specification and all implementation adhere to it, be it pandoc, sphinx, pelican, nikola. The beauty of it is that it has extension mechanisms which provides enough room for each tool to develop it. But markup can be parsed by any tool.
[1] https://docutils.sourceforge.io/docs/ref/rst/restructuredtex...