Why do Cumans lack the Heavy Camel upgrade?

I'm not sure I understand why Heavy Camels are not available to them. Is it just for gimmick reasons? Is there a balance reason?

I can't really see a balance reason for why Heavy Camels aren't allowed for them. 5% extra speed over other Heavy Camels with Husbandry isn't that broken, and there are other civs with both FU Halbs and FU Camels (i.e. Chinese).