rotisserie chickens are bred for that purpose and their lives are cut short to meet the cost and weight targets of the largest customers (walmart, etc), which means the facilities can produce 'more' in the same amount of time than roasters. they cost $6-10 here and $5-6 on sale (higher $ is at the regional convenience store chain, lower is wm), and i can frequently find 'old' ones in the cooler at wm marked down to $2 (yea, just two bucks each).

roasters are larger, priced by weight, and usually cost more (per bird) than rotisserie chicken. here, they're $10-12 at wm, $15 and up at the 'local' grocery store. they're rarely on sale.

As a general rule of thumb, any in-store prepared food in a large grocery store (the ones that come in the store's own packaging) was probably raw food that had been sitting for ages and they couldn't sell in time. They'll literally cut mold out of fruits and slice up the rest for those plastic platters. Every time they re-print a label, it resets the expiry date and you have no idea how many times they've done that and just shuffled food around by turning it into different forms.

CBC Marketplace video: https://www.youtube.com/watch?v=ZxCT_D6HBd8