Not only do meat-varieties have shorter life than egg-varieties (I think egg-varieties take longer to lay their first egg than meat-varieties get total), but there are also many more egg-layers at each facility.
Also, the locations of the facilities relative to migration routes or windstreams may be playing a role. I've heard the egg facilities are more concentrated around migration routes, so they're more likely to get contaminated.
It's a good question / interesting topic - but a terrible situation otherwise from non-academic standpoints