the vehicles do get better gas mileage than standard automobiles

Not true. Direct injection diesels commonly do 60-70MPG without any foolishness like regenerative braking or automatic freewheeling. The Audi A2 for example can do 78MPG, and the VW Lupo 3L (special eco-car) does well over 100MPG. That's about what hybrids claim but never actually manage, plus direct injection diesels are quiet, more rapid and have greater torque than petrols. It's a win win! Take a look at this list of diesels and their MPGs, or this series of articles about why diesels aren't taken seriously in the US.

Only in the US where diesels aren't taken seriously (less than 1% of new cars are diesels in the US compared to about 40% in the EU) could hybrids ever be considered with a serious face.