Do I really have to spell it out to you? Okay, since you insist.
How do you catch the flu? You get if from being in close quarters with someone who has it, while someone you never meet and with whom you share nothing and no one will not infect you. With me so far? The populations of chickens that produce eggs and that are raised for slaughter don't have contact because they're completely different industries. You don't see Tyson and Perdue on egg cartons, for example, while you do see them on packages of chicken meat. Therefore, when egg-laying chickens get sick, they pose a much lower risk of spreading the disease to chickens raised for slaughter then they do to chickens in their same industry, which can spread the disease via the trucks used to transport the eggs from farm to processors, just for one example. A truck out collecting eggs goes from farm to farm, but since the only farms it visits has egg-layers, it's not going to spread anything to farms raising chickens for slaughter.
Is all of this making sense to you?