If one goes to the Amazon site, and searches for teaching fractions and selects books as type, and selects relevance, the books that are serious books on teaching fractions have few reviews and low sales rank.
Thus we see nihilism and apathy at work. Teaching fractions is quite difficult. It is conceptually a load.
Yet the books that sell on fractions are explanation free drill books.
The demand side doesn’t demand conceptual understanding. The demand side just wants to learn the test. They want skills and drills.
This is why the supply of concept books is so low and so dismal. The demand side won’t support it.