The Constitution tried to distribute power out as much as possible -- to give power to as many different groups or people as possible. This is one of the fundamental ideas of American Government.
For example, the Constitution gave some amount of legislative power to each branch of government. The Congress gets to pass the laws, but the President may veto them if he (or someday she) sees fit to do so. The judicial branch can even get in on the act by saying that a law passed by Congress and signed by the President is unconstitutional and therefore invalid.
The Constitution also gave power to the states as well as to the federal government. This, too, was meant to make sure that there would be various centers of power so that no one part of the government got too much power.