Why do current hybrid systems employ a gasoline component, rather than a diesel component?

Diesel engines run more efficiently than gasoline engines, and, given the fact that current hybrid vehicles like the Toyota Prius or Honda Civic with their systems can not even match the fuel efficiency of way better performing, European turbo-diesels of equal size (e.g. VW Golf & Pat Tdi, Audi A3 & A4 Tdi, various Peigeots, Renaults, Citroens, Fiats, BMW 120d, etc.), I wonder why they didn't choose a diesel as a component of their hybrid systems over the current gasoline engines used in that combination. Or are they just doing the "hybrid notion" for PR purposes, and don't want to get "too efficient" anyway?