Because even though it is old the Constitution of the United States is the most well thought out document in history. The founders were even smart enough to put in a provision for it to be changed by Amending it. They were also smart enough to know that a mob often wants to change things with no regard to the unintended consequence. By requiring a supermajority, the founding fathers ensured that there would be a debate before any changes were made and that a supermajority would have to agree to the change.
