The reason is to create stability. A stable basic set of laws will allow people to make plans for the future. They won't have to worry that laws will change from day to day or generation to generation. This is why the founders made it so the Constitution was very difficult to change.
Yet, at the same time, the founders knew that mistakes would be made, and changes that would need to be dealt with, and this is why they made it so it was possible to change the Constitution, yet not easy. They wanted the Constitution to create a set of laws that would last the test of time -- like a stone.
This stone, in turn, would create stability for all the people of all generations. This was the essential groundwork that lead to the United States becoming the leader of the free world, and the inventor of the modern world as we know it today. This was all the result of stability.
W. Cleon Skousen, in his book "The 5,000 year Leap" (1981 pages 243-244), writes that "under established law every person's rights and duties are defined... This is the security which is designed to provide a high degree of freedom from fear and therefore freedom to act. Such a society gives its people a sense of liberty -- liberty under law. The American founders believed that without the protection of law there can be no liberty."
Cleon was right. Consider that in the 1930s the whims of man was toward a progressive/ socialist movement that created a larger federal government that would take from those who have to help those who have little. This resulted in a long recession, and has culminated to a nation pent on spending more than it earns.
This has resulted in an unstable economy where people are unsure of what to do, and are unwilling to invest time to study new inventions, or the money to invest in expanding business that would create new jobs. This was the result of the liberal movement in America.
Most of the problems that ail America today are the result of some people over the past 100 years viewing the Constitution as an evolving document that changes with the times, as opposed to one etched in stone.
By ignoring the Constitution, or by finding a way around it, many laws and pseudo laws have been created that have resulted in instability in the United States. It has resulted in an unstable economy, the falling value of the dollar, declining values, and a great partisan divide.
Examples are many:
- Rowe-V-Wade bypassing the Constitution to make abortion legal (this was a decision that should have been left to the states to decide)
- Obamacare (the 10th ammendment was ignored)
- The 13th ammendment (made it possible to tax Americans unequally, and punishes achievers)
- 17th ammendment (took away the state's rights to check the federal government)
What sounds like a good idea today may not end up being a good idea for the future, and if liberalism/ socialism has taught us anything this is it. The cost of making decisons based on a whim and a new idea have affected our nation greatly.