Because the constitution is setting up the rules for what can completely superceed it. Basically: Congress and the President must follow these rules when governing the country, except when they agree not to, as declared in these same rules.
I get the need for treaties and and federal laws to superceed local laws, but that laws have to be made pursuant to the Constitution while treaties only have to be made under the authority of the United States, defined as the President and 2/3 of the Senate, is odd. Seems as though they could legally eliminate all constitutional obligations through treaty, or even enter into a treaty to use a different constitution.
http://en.wikipedia.org/wiki/Bricker_Amendment#Legal_backgro...