Why does the hatchery purposely keep some embryos from developing high intelligence? in brave new world?

The world controller orders the hatchery to create embryos with different intelligence levels to fit into different sections of society. Alphas require a lot of intelligence to perform complex activities like Bernard's job of overseeing the night-time conditioning of the young. By comparison, the lowest cast, Epsilons, are deprived of oxygen during embryonic formation and given alcohol to ensure they will have very little intelligence and thus be fit for the lowest jobs in society.