I really think this is an important question these days, in spite of its sometimes being asked simply as a pretext for a particular answer.
Here's my take on the question. Basically it comes down to original sin and the total depravity of man. This is just basic Christian doctrine that says (among many other things) that Man Cannot Be Trusted.
That's pretty much all it takes to understand why the Constitution is set up as it is - to make sure that the federal government cannot have too much power over the States or over the People, and to make sure that each branch is answerable to the other two. Further, the right of citizens of the United States to move from State to State was an additional protection from despotism.
The framers had had some recent experience with individuals having relatively unlimited power, and they realised that it was not a terribly good idea. Even if now and again there might be a monarch, dictator, ruler, leader, or whatever with whom they agreed, more often than not it would be the other way round.
There's the saying that power corrupts, and that absolute power corrupts absolutely. I think it works the other way as well - when a corrupt person takes power, he corrupts the power he's taken. Given the Christian understanding of human nature, the limited nature of federal power is something for which to be grateful.
