I think the idea/rumor/urban legend of 12R22.5s being hard to find a truck stops comes from the early '70s, when the charter, tour & line-haul buses first began running this size.
At that time, 24.5s were the most common truck tire rim size, so naturally were abundantly stocked at most shops.
Obviously that has changed in the ensuing 30+ years. And, with the improvement in tire technology over the same period, that leads to:
Gary hitting the nail right on the head as a reason for the smaller tires on the 18-wheelers: they weigh less, which means the truck can haul more weight, which equals more revenue per load. Follow the money!!
FWIW & HTH. . .