The influence of religious and conservative types has weakened US textbooks. Do a deep dive into how Texas can dictate what is presented in schoolbooks nationwide.
There are plenty of south/East Asian counties that have just as much if not more restrictions in textbooks. So I don’t think that alone accounts for the discrepancy
Be careful about reading and comprehending schoolbooks in Texas. Parents don't like to be pressed on most of the content, and are sometimes going to great lengths to get your book banned entirely.
That story wouldn’t work in deeply secular England, & it seems unlikely to explain underperformance in maths.