Meat birds have from hatching to butcher is 8-10 weeks I believe. And in grower houses there may only be a few thousand in a barn, I’m guessing a grower farm may have 10,000-50,000 birds total.
So if a grower farm gets hit with AI the number of birds isn’t that high. Also once those birds are killed and the cleaning process is complete and repopulation happens those birds grow so fast before they are slaughtered. (Maybe 6-8 weeks on the farm.)
With layers a chicken won’t start laying eggs until they are 20 weeks old. And they lay until they are around 80 weeks old.
And a small later farm has around 1 million birds. One farm I’ve worked at has 6 million birds.
So between the sheer number of layers that need to be killed is way higher than meat birds. Along with the much lengthier process to get back to production. If a layer farm gets hit with AI it takes them roughly 2 years to get back to full production.