It's also worth remembering that markdown tried very hard to encode conventions that were already used in Usenet, email, and other text media. A > to indicate a quote was widespread Usenet convention. Asterisks or underscores to indicate emphasis was also a common convention; both are legal because both were common. Double asterisk or double underscores to indicate really, really emphasizing something was also a common convention. So were asterisks to display a bulleted list, blank lines to separate paragraphs, and indenting 4+ spaces to write code.

It's a good example of "pave the path" design philosophy, where you do what users are already doing rather than trying to impose some platonic ideal of what the world should be like. And it works quite well at that.