Quite simple -- the additional weight mandated by the safety requirements.
I remember when there was one small UK car -- I think that it was the MGB for a certain year or two -- where the requirements caused it to need three windshield wiper blades. The requirement was that a certain number of square inches of windshield *must* be cleaned by the wipers -- and the whole windshield did not have that many square inches. So -- the solution was a third windshield wiper whose coverage overlapped the other two -- but the sum of the wiped areas of the three was sufficient to get it past the requirements. :-)
With things as idiotic as this -- how can people make cars light and small enough to be energy efficient?
Enjoy, DoN.