Republics tend toward centralism. The United States was conceived as a nation of states, but democratic pressures have driven us toward uniformity. This has far-reaching results. The Civil Rights Acts ended one kind of regional diversity. Roe vs. Wade, the abortion rights Supreme Court decision, ended another. Escobedo and Miranda changed the criminal law for every state. Congress is about to undo some of that with the new Drug Law, but the states aren't being consulted. The National Defense Education Act effectively federalized the education system.
The same trends dominate within states. Local school boards are made powerless while state bureaucracy multiplies.
Some will see all these trends as simple justice. Others see a centralized government riding roughshod over local differences. A few will even see the tyranny of the majority that Tocqueville warned us about.
Majority decisions are usually compromises. Uniformity imposed by a majority has the advantage that most of the population—by definition—accepts the national decision. Problems arise when a powerful minority truly believes the majority decision is immoral. In such cases compromises are impossible: how can you compromise between a group that believes abortion is murder, and a group that says that anything less than full free choice is slavery for women? Even so, the pressure for uniformity is overwhelming.
Empires have traditionally preserved diversities. So long as the subject states pay their taxes and hold allegiance to the Emperor, they may have whatever local customs and laws that they like.
Of course some local customs are more important than others. So are some of the subject states . . .