"Majority rule" is a simple, understandable message. There must have been convoluted arguments when creating the "super-majority" dividing line.
First, there is the problem of defending why there should even be a super-majority. Next comes the gargantuan task of describing where a dividing line would be. It could be 2/3, 4/5 or even 9/10. It has changed several times. If the goal is to show bipartisanship, why not pick a crossover rule of "majority +/- one"?
Even perfect bipartisanship is unworkable. If 25 Democrats voted with the Republicans and 25 Republicans voted with the Democrats, there would still be no progress.
The original discussions must have been loaded with loose facts, loaded opinions and obvious power grabs, resulting in an arbitrary line in the sand. There is nothing "logically or ethically right" about a 60-vote rule. Discussions/debate in 2021 would be the same.
It make more sense to use the "majority rule" guideline. Voting citizens then have their say in the following elections.
Societal change does not follow a straight-line development. There always has been (and will in the future be) a ratcheting between progressive and conservative thinking (and voting). It takes time and reality-testing to develop an improved society.