"The Royal Society of Chemistry said that as maths was a difficult subject, schools feared examination failures which would threaten their standings. Chief executive Richard Pike also said universities were increasingly having to run remedial classes in maths.
. . .
Dr Pike said: 'Schools and students are reluctant to consider A-level mathematics to age 18, because the subject is regarded as difficult, and with league tables and university entrance governed by A-level points, easier subjects are taken.'"
I don't have a background in mathematical education but was an applied mathematics undergrad at UConn where there were a surprising number of "math-ed" students.
A quick search through the AMS archives found this collection of articles on mathematics education in NOTICES of the American Mathematical Society.