Aside from the merits of Markdown and Asciidoc, Asciidoc has a name that doesn't help. Depending on the (natural) language, ASCII (the encoding, that is) annoys anything from a small percentage of users (e.g., English, Esperanto), to many people (e.g, Dutch, German, French), to almost everyone (anything that doesn't use Latin characters). In the context of a markup language intended for structuring normal text its name just screams obsolescence for a lot of potential users!
Absolutely, or at least I was also put off the name when I first heard of it. The name sounds like something that requires you to use (HTML) entities or some kind of Unicode code point syntax if you want to input non-ASCII. Blegh…
It might not occur to people in North America that even British English users are inconvenienced when restricted to ASCII only (GBP vs £) and therefore the name implies that this tool is not for us.
Along with the notion of obsolescence one might derive from it, it's also not particularly sticky/memorable. It might sound silly but I think snappiness or lack thereof has an impact on rate of adoption, because for people to use things they have to first remember them.
