Regulation is a key driver for towbar design. In Europe, ECE 55 defines most aspects of towbar design; from the height of the ball in respect of the ground, to the durability testing of the assembly. It runs to 72 pages.
In America they have SAE J684, which has similar aims but a lighter approach. It is 7 pages long.
The US standard defines an under bumper receiver, a hitch system. The detachable towballs are manually fixed into the receiver with a hitch pin and a locking device.
In Europe there is no prescribed receiver shape or orientation. This has spawned a multitude of fittings. The US system is deemed not safe enough in Europe.
In the US the open standard of the hitch receiver has spawned a new industry, of steps, load carriers and bike racks.