[sottol]

Afaik, it has partially to do with regulations. In Europe the max length of a truck includes the cab, thus it's as short and upright as possible to leave max room for cargo (cab over engine), whereas US trucks have the long "hood". Old trucks were quite angular but the new ones are a lot more aerodynamic.

Second, those "tapered" add-ons in the back can help a lot to approximate a teardrop shape better given that there are a lot more constraints in general in the front.