the answer to 99% of things in this country is "money."
for the poorest kids, that they don't get the same level of infrastructure, teaching, coaching, as private school or suburban kids, which also drags down the averages.
for the rest, that ever since about reagan all this place seems to prize is money and schemes to make it. despite consuming stuff that reflects other values, eg, religion, academia, art, music, a lot of what is put out there to consume is to make a fast back.
my theory on american class structure is the wealthy are free to pursue knowledge and major in what they want, with fewer worries about a job at the end, and thus may be, say, a yale history major (one of the most popular majors there). meanwhile the middle class and poor approach life as "i need a job." so the state colleges for the middle class are actually cutting back on that same history major to fund business departments and such. and the poor often aspire to a trade as a stable job is better than being broke. setting aside the resources discussion i began with, i think the class analysis points at another reason we suck at global academic comparisons. the middle and lower classes do not value a broad sense of academics. they see school as a tool to a job. they do enough to get to the job/money hamster wheel.