Cost Of Dominance
It has been suggested that in many circumstances being dominant is no more advantageous than being subordinate. For example, a dominant male may obtain a territory which is more resource-rich than that of a subordinate, but it may have to spend much more energy to protect it. Therefore, costs and benefits may more or less balance one another at each level in a dominance hierarchy, and evolution may favor the maintenance of the hierarchy itself, rather than just those near the top (which could lead to the disappearance of differences in dominance).
Source: Stanford UniversityCost Of Dominance