It is due to business. The cost of raising chickens is in the chick and processing and retail packaging. The profit is putting on weight. Smaller chickens maybe sold as parts, and to restaurants and delis.
Costco maybe the largest purchaser of ~3# chickens.
I am in Silicon Valley, a local grocery with a butcher shop, will reserve a 3# chicken for me, otherwise, the smaller chickens go to the deli or are parted out.
