I think it mostly comes down to this, how hatcheries are there. The U.S. is a HUUUUUGE country with a lot of demand for chicks, so our hatcheries tend to put quantity of chicks hatched over the quality of those chicks. In other words, a hen that lays a lot of eggs that can be hatched is priority over a hen that looks great. Many decades of selecting for productivity has led many of our hatchery strains to adapt a visually similar body shape, one adapted to high egg production, with the bulk of the visual differences between breeds lying only in feather color. This is where the infamous hatchery Buff Orpington comes from.