Jodi, this is just my guess. But aside from the annual moult which often causes them to temporarily stop or slow down laying, a lot of the production breeds just aren't, well... designed to keep laying for years and years. This is because the people who breed these commercial layers are only interested in the first year or two of the hen's life, not the overall productive life of a backyard hen.
Again, I would guess that a bird who doesn't get burned out early on by genetic factors, and whose diet and environment doesn't gear it only for hopped up egg production probably has a much longer productive span. For myself, I'll be much more interested in having a hen who can lay 3 eggs a week for 10 healthy years than one who can lay 5 eggs a week for two years.
Commercially, that would be a big financial hit. For a backyard chicken farmer, not so much.