Markdown was originally a loose formalization of how people already wrote text only emails. It was how _people_ wrote, not how to optimize for the computer.
Maybe Markdown just hit the sweet spot. But I actually suspect it's something else: as soon as it's very convenient to have machines ingesting and producing a language, it becomes tempting to create tooling that is unnecessarily strict in what it will accept while at the same time spitting out markup that is unusually difficult for humans to read. A sort of specialized violation of the Robustness Principle.
The most famous example of this was probably XHTML, but even HTML5 suffers from it. And so we end up with stuff like Markdown because HTML tooling is too hostile to work with directly, and AsciiDoc because DocBook XML is similarly awful. They don't have to be, but...