Some of it is cultural: Americans don't want their kids to have too much homework, in many cases they don't want to help with homework or are unable to; they want a silver bullet. Some of it is lack of qualified teachers, and part of that is the way public schools are funded from the local tax base--do it cheaply as possible. Some of it is political pressure to dumb things down so that students can be graduated from HS before they reach the legal drinking age. Some of it is the rise of the education ``experts'' who always have some new…silver bullet (Feynman had a lot to say about such experts after reviewing math textbooks for California in the early '60s). The new math came about as an almost hysterical reaction to Sputnik, when, in fact, the U.S. had plenty of highly qualified scientists and engineers who just happened to get beaten to the punch by the URSS. But no, the math curriculum had to change into some kind of Bourbaki for tots, meanwhile in fact the Soviets were teaching math the old-fashioned way--nice cognitive dissonance there. There are a lot of factors, and I don't see any magical way of improving curricula, getting better teachers, requiring more homework, etc.