Because they can be made fun of for their stupidity, ignorance, or vices and what he said above, a good example of this is the comedians at the White House Correspondents yearly dinners, for example Stephen Colbert on George Bush. He grilled Bush really bad.
Our Prosperity refers to the children and descendants of the writers of the Constitution. They wanted to emphasize that Prosperity was the Americans who through the coming years would benefit from the Constitution.