The Framers of the Constitution of the United States set the government up in this way because they did not want a government in which it would be easy for radical and perhaps oppressive changes to be made. They wanted a government in which it would be difficult for major changes in the law to be made.
The Framers were very worried about the possibility that the government would oppress the people. They were also worried that the government would do things that were very ill-advised. For these reasons, they wanted a government that would be somewhat unwieldy. They wanted a government where multiple, independent groups (like the two houses of Congress and the President) would all have to agree in order for changes in the law to be made. If multiple groups of people had to approve of each law, it would be less likely that an oppressive law would make it through. It would also be less likely that really bad laws would be passed because it would be much less likely for many people to approve a bad law than it would be for one person or just one body of people to do so.
In short, then, this system was set up to prevent the government from doing anything very easily. This was meant to protect us from oppression and from bad decisions on the part of the government.