Suppose that, one day in the future, there is a national referendum. Eighty million people vote “Yes,” and 79 million people vote “No.” In this case, the answer is “Yes.” If our decision rule permits the “No” of a minority to prevail, then this grants the minority disproportionate power over the law. Equality over the law demands a simple majoritarian amendment process. It is that simple.
Critics will say that we need a supermajoritarian amendment process to protect the people from themselves, but this is not true. A majority of the American people will never vote to overturn our constitutional core of equality: regular elections, the use of the census in apportionment among the states, the First Amendment, and the equal protection clause. And supposing that the Constitution guaranteed the direct election of the president, proportional representation in the Senate, and amendment by simple majority of the people, a majority of the American people would never vote to return to the Electoral College, equal state representation in the Senate, or Article V. Never. A strong majority of the American people have a good sense of fairness. The people will protect the proposition that all people are created equal.
This is the truth regarding most constitutional supermajoritanianism and prohibitions on constitutional change: it is and always has been about protecting inequality over the law, and it is a very direct relationship. Article V includes an absolute prohibition on amendments which would interfere with the slave trade before 1808, or which would allow slaves to be taxed out of proportion to apportionment. Article V seems to mandate unanimous state consent to end equal state representation in the Senate. The “three-fourths” of state legislatures or state conventions necessary for amendment ratification according to Article V was the natural result of the imperative to protect both slavery and equal state suffrage in the Senate.
The U.S. Constitution is the most difficult in the world to amend. The Swiss and Australian Constitutions are nearly as difficult to amend, and they both have similarly malapportioned Senates. The German Constitution is a little easier, but still has enough of a supermajoritarian amendment threshold to protect the design of its malapportioned Senate. Democratic nations with constitutions formed in the absence of slavery and in the absence of malapportioned legislative bodies have universally adopted easier amendment procedures.
Supermajoritarian voting rules inherently violate equality over the law and mostly exist only to perpetuate other violations of equality over the law.
See here to read about how the Founding Fathers supported majority rule.