I love AsciiDoc and I use it wherever I can. I'd bet the reason for using Markdown has more to do with mindshare than anything else. Plus, Pandoc is a great tool for converting Markdown to anything else.

What I really like about AsciiDoc is that it's right near Markdown in terms of simplicity, but with a bit more beef behind it to handle more complex things like referencing figures. If you don't need them, you can stick with the basic syntax, but the power is there when you want it.

Those tend to be pretty markup heavy, which can be annoying and distracting when actually cranking out prose.

Markdown is limited, but it's very lightweight and doesn't break your flow. It's a piece of cake to type a couple of "*" when you want to italicize something.

Also, the limitations are a benefit because they keep you from getting distracted by the hundred different ways you could structure or format a paragraph.