I could be off here, so dont get mad :P but i believe it is because of the rest of his kit, his Q does a good amount of damage but isnt a huge contributing factor in fights other then a slow and a passive stack, his W, while it does really good damage, can only realistically be applied once in a fight due to cooldowns, and his E isnt a damaging ability. The passive on his R allows him to be able to contribute more to a fight then a slow and one devour. this is especially important in a solo lane because most tanks are DOT or relative low cooldown damaging abilities, but Tahm doesnt so the passive allows him to trade more efficiently.