Not so much the fragility, this seems to come from 'production breeding' where I would guess that only the best layers are bred from and if you keep doing this, perhaps that leads to other issues. If you go and buy a Light Sussex POL from a bird dealer it is likely that it has come from an East European hatchery and the parents would have been selected so that there is some way, probably in the feathering that the chicks can be accurately sexed at hatch. They won't have been selected for longevity, good foraging ability or resistance to disease. They will also bear only a passing resemblance to a proper example of the breed.
But generally breeds are crossed to seek 'hybrid vigour' where they produce more eggs or more meat in a shorter time. Happens in most cattle / sheep etc.