Because the traits beneficial to meat production (like growth rate, muscle development etc) are inversely correlated to the traits beneficial to egg production

Because the traits beneficial to meat production (like growth rate, muscle development etc) are inversely correlated to the traits beneficial to egg production (feeding efficiency, eggs per year, egg size...). Part of that is that the animal only has so much energy to spend, the other is that when breeding for one trait, one invariably tends to neglect other traits.