When governments attempt to exert too much central power on a sizeable nation, it rarely ends well. Of course, for a nation to survive, there must be some amount of centralized power to keep the people united. In a nation the size of ours, however, too much power from the federal government (now accompanied by the diminishing of power held by states) could ultimately bring the United States down.
This phenomenon can be seen time and time again throughout history: Rome, the Third Reich, the Soviet Union and, even now, the European Union. All were great powers with great influence when they were at their peak. Each began to collapse when territories became too large to manage, and when governments attempted to rule these territories with strong central governments.
The United States by itself is 96.6% the area of Europe as a whole. Just like it would be foolish to expect someone living in southern Portugal to live based on the same laws and social standards of someone living in Kazakhstan, it would be equally as foolish to expect someone living on the Mexican border in Arizona to live by the same laws and social norms of someone living in upstate New York. This is not to say that the United States should revert back to the loose ties implicated in the Articles of Confederation, but based on the sheer size of America alone, it is obvious that more power should be held by states so that the interests of citizens living in those states are better addressed.
Returning more power to the states will hopefully allow state governments to work more efficiently in creating laws addressing the needs of their constituents as well. With less competition for federal legislation among the states, state governments should be able to focus more on the needs of their state rather than trying to please the nation as a whole. Strong central governments are good for small nations with a homogenous people.
The United States is the third largest nation in the world, and our population is diverse and our interests varying. For our nation to thrive, and for the people that make up the population to remain happy with the government, the needs of the people must be met, and the only way to properly do so is by returning power to the states.