I don’t know about the rest of the world, but I was way ahead in math in the first and second grade (in fact, something I didn’t learn until I was in my thirties, but my school considered me for double promotion from first to third grade, but decided I was too immature). Then I transferred to public school, and math was all about switching to metrics. We didn’t learn metrics, though, like estimating sizes and distances in metrics, so we’d get a “feel” for it. Instead, we did endless conversions, which required learning formulas and the decimal system. It was exhausting. It’s no wonder the US rejected it. What was being rejected was having to do a math problem every time you encountered a number or quantity like weight or size. Ever. Which is what the metric system seemed like in the third grade.
We had barely learned the standard system. If they had given us problems like “A cat should be weighed in a) grams; b) kilograms; c) micrograms; d) metric tons;” or “A loaf of bread is approximately a) 30 centimeters; b) 1 meter; c) 15 centimeters; d) 30 millimeters,” we would have learned to think in metrics, while conversions kept our minds on the standard system-- not to mention that calling it the “standard” system didn’t do metrics any favors. I was pro-metrics when the process began, and anti when the month-long math unit ended. It put us behind the previous third grade on division, too.
So anyway, that’s the 1970s.
Why is math still hard?
I don’t know. I do know that I can do complicated arithmetic in my head, which is first and second grade math, and a little more elementary school math, but I get totally lost and wonder if I have dyscalculia when is comes to anything beyond algebra.
I also know in Europe they teach algebra in the lower grades, and we don’t introduce it until high school. My getting lost in math pretty much coincides with puberty.
Why I didn’t get equally lost in other subjects? well, English teachers were smart enough to cater to preteen angst, and I did get lost in science, but it was easier to catch up, and I had a father who taught political science who did not allow me to get lost in social studies. Other than that, I don’t know. My grades were not spectacular then.
I think the issue with math, unlike some other subjects, is that, once lost, lost forever, unless you go back and remediate what you missed.
Other than that, I have no guesses.
TL;DR: Teachers make it hard, and once lost lost forever, unlike other subjects.