The eldest son isn't automatically the heir, that's just a tradition that isn't always followed. Liu Biao wanted Liu Zong to inherit Jing, not Liu Qi, he doesn't have the benefit of hindsight like we do. Therefore, the rightful heir is Liu Zong.
BTW, giving the land to Liu Bei isn't any better than giving the land to Cao Cao, since they weren't in any way related other than having the same last name...


