It’s in the holy text: the markdown standard.

Ninja edit: the reason is because a single line break should have no formatting effect so that markdown files can be opened and show up better in terminal editors like vim or emacs. It’s good to be able to break a line when it gets too long in editors which don’t use page formatting to handle line wrapping (ie. like MS Word does). So you need something other than a single newline character to represent a rendered paragraph break.