Why might the delegates to the Constitutional Convention have made the Constitution so hard to change?

So some nobody can't just go up and change the laws, even till this day not any one can change a law or two. You must be in a higher position

So that the common, uninformed public could not make a "spur of the moment" descision that would radically alter the very foundation of our government. Also, so that a select group of people could not completely control the government. That's why the Constitution requires the majority of the people in order to alter it.