It's local homeowner associations, the tendency to litigate everything, the inability for large actors like government or business to purchase and develop land. The one person who symbolizes the reason why modern America can't build for me is Erin Brockovich. A person without any formal training in the legal field suing an entire project into the ground, although as it turns out there's actually no scientific evidence for any of the claims, yet she's celebrated as the little guy who stuck it to the man.
Devolving power to the states may only weaken the federal government whose resources and knowledge are needed to provide large scale infrastructure. The deficit here isn't in the billions, its in the trillions.
I'm not in principal opposed to experimenting with local democracy or whatever but it needs to happen on the back of a federal government with sufficient capacity and competence and power to act quickly.
For the "states as laboratories" thing to work you actually need a mechanism to ensure that the things that work actually get adopted. I don't see this happening in the US. There's way too many inmates running their own asylums.