Just an observation here, with no data to back it up -- Most of the folks buying compacts and sub compacts are moving into the tractor ownership from the lawn mower, lawn "tractor" and garden tractor perspective. They are possibly into cars or trucks pretty heavily. From that point of view, R4 tires look all macho, rugged, and unstoppable. Hence, when they buy or order a CUT, they go with the R4 tires. Since these guys make up a very large part of the customer base, R4 tires tend to look like the item to order to the guys in the sales department.
On the other hand, those who are coming to CUTs from the bigger tractor end will often want maximum traction and are not too impressed by macho looks. They know what works, and that's what they buy.