I guess it's because North Americans like their trucks big. Seems like Ford would be working on a line of small, fuel-efficient trucks rather than going with the larger one, but it may have made more sense to them to close out a defined portion of the business.
It says to me that they don't really think that fuel prices will affect the market in the long run. Toyota apparently also believes that with their Tundra.
I still remember seeing a German couple with their Euro-spec Nissan diesel, getting low 20's with their camper body on it.