The beauty of our American Democracy is that our right to vote is protected by the Constitution. The ability amend the Constitution ensures that it is not a static document.
The credentials (or lack thereof) for voting ensure not the intelligence of the voters but the ability of citizens to participate in the democracy that governs them.
Credentials for holding office also contribute to the eligibility of as many people as possible to participate in our democracy but do not guarantee good policy or the most qualified people representing us. The benefit of our existing system is that we can unseat existing office-holders when we disagree with the policies and then we can convince voters to give us the chance to make better policy ourselves.
Changing the system to ensure intelligence in voting and representation will more than likely begin the slippery slope to an elitist government and eventually some form of a dictatorship.