As far as legal, historical, and constitutional reasons go, the original idea of the US was sovereign states under a limited federal government. The federal government inherently had no power except that explicitly granted to it by the constitution. Several part of the constitution (most notably the commerce clause) have been stretched far beyond their original meaning and purpose to make the federal government far more powerful than the states.
As far as moral reasons go, I've personally always been of the opinion that government should be as close to the people as possible. National governments (and international ones like the UN and EU) make it almost impossible to truly hold elected officials accountable.
That's history, but not reasoning. The constitution adapts to the people's needs, I don't believe we've gone down the "wrong" path, but either way the constitution is neutral towards the debate.