Another reason is that longer trucks are harder to overtake on motorways, the longer the truck the higher the risks for accidents. Europe has smaller highways and country roads.

Furthermore truck logistics in the USA is mainly based on FTL (full truck load), which means the truck just drives from A to B. In Europe LTL (less than truck load) is more common, which means pickup and delivery on multiple stops. This requires a better maneuverability, hence shorter trucks.