The Constitution of the United States was the first written national constitution. Being first does not necessarily mean being the best.
The men who wrote the Constitution of the United States in 1787 were only charged by the Confederation Congress with the task of reforming the Articles of Confederation. A new constitution was written because those men thought that there was no reform of the Articles that would be sufficient to meet the needs of the nation. They decided that no higher permission or authority was needed for them to create an entirely new structure for national government.
Since the writing of the Constitution had not been authorized by the Confederation Congress, it did not seem appropriate for the Confederation Congress to approve it, nor even for approval to come from the state legislatures. As the opening words of the Constitution declared, "We the People of the United States," it was decided that the Constitution would be ratified through ratification assemblies whose delegates were elected by the citizens of each of the states.
If the time is not right, certain ideas might not even occur to people. The idea of a national referendum as the means for ratifying the Constitution was unthinkable in 1787. It was also unthinkable at that time for women to vote, and certainly not slaves. In some states, even if one was a white man, if he did not own property, that man was not allowed to vote. We can do better today.
One way we can do better is through universal suffrage; respecting the right of every citizen of planet Earth who wants to vote to exercise that right, and to vote in a global referendum.
Another way we can do better is by focusing on principles instead of structure. The Constitution of the United States, as ratified, was about structure. It spelled out the position and the duties of the President, and of Congress, and of the Courts. The framers chose not to include a Bill of Rights. The first ten amendments to the Constitution were approved because there had been widespread opposition to ratification without a Bill of Rights.
How does that history inform our present actions, and what are the principles which should guide our relationships with each other and with the rest of the planet today? What better principles are there on which we can focus, and deliberate, and act than those expressed in the Universal Declaration of Human Rights and the Earth Charter?
If the principles in those documents were given the force of supreme law for humanity through a global referendum, would any further structure through a world constitution be needed? It would be the job of existing local, state, national, and international agencies and courts to enforce the principles. The basis for the enforcement would be the will of the people, raised to a higher level of awareness through the period spent in deliberation before the referendum took place, and through constant monitoring and insistence on enforcement after the referendum.
That might be pie in the sky. It might also be keeping one's eyes on the prize.
We can learn from the past to do much better today and tomorrow.