I think it is because of a fairly administered salary cap. Every team spends the same (more or less). The proof is in the pudding, 5 different champions in the last 5 years.
A buffalo herd can only move as fast as the slowest buffalo & those are killed first. This natural selection is good for the herd as a whole because the health of the herd keeps improving by the regular killing of the weakest members. Similarly, the human brain can only operate as fast as the slowest brain cells. Intake of alcohol, killing the slowest & weakest brain cells first, makes the brain a faster, more efficient machine. That's why you always feel smarter after a few beers.