There's no point in having different content formats in the notes because any client can display Asciidoc content however they want. It would be redundant information.

Alexandria displays them as HTML because it's web-based. (There is no such thing as displaying markup in a browser, just HTML.) A client can generate an ePUB, PDF, LaTeX, txt, doc, etc. document from the events and the author can stipulate which stylesheet to use, in that case. But editing that document would have no effect on the state of the event it was generated from; it's a snapshot. The Nostr event is the Single Point of Truth.