Federalism is a complex topic. Some things are done better In a central way, and some are done better distributed.

Uniform regulation of commerce and military protection is really efficient.

At the time, there was no practical way for one body to make meaningful policy to manage both new York and south Carolina at the same time.

You can basically look at what the EU is doing and that's why the states did it too, just starting with "shit we need money, a navy and soldiers" rather than "can we all just agree on food standards and currency?”