The founders made the US Constitution flexible because they understood that American society would change over time and that the laws would need to adapt to the time. For example, the problems faced by Americans in the 1700's are much different than the one's faced by Americans in the 1800's.
By allowing Congress to change the constitution as needed and giving the ability of the Supreme Court to interpret what the constitution means, the Constitution becomes a document that adapts to American society as needed.
