Probably because asciidoc recommends asciidoc-dblatex (used for producing LaTeX via docbook). Try 'apt install asciidoc asciidoc-dblatex-' to install just asciidoc without pulling in the other package.
apt --no-install-recommends foobar
"The requested action can be overridden for specific packages by appending a plus (+) to the package name to install this package or a minus (-) to remove it"