New math was actually pretty successful at the lower grades. The issue was that when it came to rolling it out to middle and high schools, they just kind of said "here's the textbook, figure it out" instead of going through a coordinated process of teaching the teachers while helping them develop their new curricula (which they did for elementary school).

So you had this really ugly failure of teachers not really necessarily being prepared to teach the material combined with the rush to roll out the curriculum across the board instead of expanding it year by year so there wouldn't be a sudden change in expected education.

I have a theory that the enthusiasm of teachers teaching the material is a far bigger factor in the effectiveness of learning than the methods. So much so that any advantage from better methods gets quickly nullified without teacher buy in

Every single teacher I remember as being influential on me in any significant was was hugely enthusiastic for their subject and the material. No matter how hard or easy the class was, that enthusiasm was definitely the biggest contributing factor to how much of an impact they had on me. One of the only classes I ever really struggled with was a government / civics class. The teacher assigned difficult work and graded hard. But they were enthusiastic, firmly convinced that the key to understanding US government and politics was understanding the sides and arguments in the major landmark Supreme Court cases. So convinced of this were they, that many classes were spent with them enthusiastically recreating oral arguments for various cases. Presenting both halves of them as if they were the lawyers, and leaving us to ask questions about the positions and the arguments.

To this day, that enthusiasm for those cases, for understanding both how each side of these cases is both convinced they’re in the right and how often the cases pivot on very narrow aspects of the law has carried over for me decades later. Those lessons and the insights have shaped not only my passing interests in law and politics, but how I approach day to day conflict and debate.

If they had been unenthusiastic and dry, like so many other teachers I’d had, theirs would have been just another boring US history class with a jerk of a “difficult” teacher.

I have an anecdotal theory that most people who came prepared to a STEM programme in the States last century did so despite, not due to, the modal 7-12 teaching. (if true, I hope it no longer is?)

Most teachers in the states did poorly in math, and never loved it. They in turn can't pass a love of math on. They are good enough at math to pass the tests, but that isn't why they are teaching.

That's why the US can get stuck in such educational ruts. There are too many teachers who don't have the in-depth understanding to alter their teaching methods to approaches other than the one they memorized in education college.