While that all makes perfect sense, weight distribution has nothing to do with it, the axle weights are the same no matter where the cab sits on the chassis.
Bridge law doesn’t factor either, that is calculated from the kingpin to trailer axles, different lengths in different states and different points of measurement on the trailer axles, but nothing to do with the tractor unit at all.
The conventional took off when they removed overall length laws, in the US at least, Canada still has overall length laws and a maximum wheelbase of 244" so you don’t see many big sleepers there.
The two biggest factors in choosing a conventional over a cabover are ease of access to the engine for routine maintenance and driver access, believe it or not but claims to workers compensation for drivers falling out of their cabs was getting to be a huge cost to the industry, the average American is not a small chap and quite a lot of them had big problems climbing into a cavover, not hard to believe when you look at the step arrangement caused by the need (axle weights) to have the front axle mounted so far forward.
The other reason and probably the most likely is purely down to aesthetics, the long nose big bunk classic truck is a thing of beauty and that counts for a lot over here.