The problem is described in the scriptures this way:
We have learned by sad experience that it is the nature and disposition of almost all men, as soon as they get a little authority, as they suppose, they will immediately begin to exercise unrighteous dominion. D&C 121:39
"Almost all" is pretty inclusive. Too much power in one place invites the worst in people. That's why I would rather power be decentralized to states, counties, and municipalities.