Ford used to make some cab-over semi trucks that were very popular in the US during the 80's and early 90's and resembled very closely the European designs of today.

I think it's pretty obvious and comes down to a few things:

* European trucks typically run shorter hauls, and do not require a bunk in the cab as often as multi-day cross-country runs.

* European Semi's need to accommodate all countries' roads and parking lots. This means the much wider range of nonstandard roads, docks, fueling stations and parking lots extends much farther towards the "small" side. Tight streets and parking lots - making it necessary for the truck to be shorter for tighter turns and parking compatibility.

* It has been proven that a cab-forward design is safer for avoiding accidents directly in front of the vehicles, something that is more focused on in the EU.

* The least obvious but possibly the most relevant reason why it remains: Having and ingraining differing standards makes it harder for a single competitor to play into the both markets. EU is largely locked down by EU manufacturers since any US manufacturers would have to significantly change their design and production to sell there.

The fuel economy point seems to be a distant consideration since the majority of drag economy comes from the trailer and not the tractor.