I would hazard a guess it has to do with the availbility of cartridge bearings vs the manufacturing of a special bearings and races and the labor to make it all smooth.polished and slick. Much easier for a manufacturer to lathe out a square surface in some aluminum and order a cartridge bearing from a catalog vs making a shell, forming it correctly, grinding and polishing it smooth, pressing it into a hub, order or making the custom inner race. making a custom bearing cage, then put it all together with that gorgeous looking axle....
But thats just mho..