That may be because of the vicious cycle of lack of popularity, which leads to worse tool support, which leads to people preferring Markdown-based tools, which leads to increased spread of markdown and less popularity for AsciiDoc.

One lesser known fact of AsciiDoc is that it's a markup syntax for DocBook, so it supports the full feature set of this book-creating toolchain. Markdown can only dream of having support for more than simple web pages and single-page documents.