There is a convention factor also. People who want carbon frames may want to merely have the degree of change substituting materials in the tubes will create. Going to a monocoque is a whole other discussion.

Monocoque is a good way to build certain structures where surfaces and structure are in close alignment, But in other cases a less zowie approach may work out better. Compare the all carbon wheel, to the equal quality conventional wheel with a lightweight fairing over it for aero. You can actually be better off with the rag and stick approach of the latter, compared to the aero carbon wheel, when there isn't a regulatory bias agains fairings.