The basic fallacy of the U.S. is that, because we speak a single language*, we have a common culture. Nothing could be further from the truth.
* I see no evidence that significant numbers of children born in the 50 states don't learn English.
The U.S. is a multinational state, with dramatically different cultures. As a child of immigrants from Ireland who grew up in the Northeastern U.S., I would be far more socially comfortable in Ireland, Greater London, or New Zealand than I would be in west Texas or North Dakota.
Because we deny these dramatic differences, our assumptions of how to structure a government are fundamentally flawed. It is hard enough to make social or tax policy in Maryland, where 90% of the population lives inside a circle 60 miles across. Trying to set more than broad standards for southern California, Maine, and Texas from Washington is insane.