John Saxon was an algebra teacher who was disappointed in the results he was getting with the standard texts. After studying older texts from earlier in the century he noticed that after introducing new mathematical concepts they would constantly review those concepts before giving the kids new material to master. And that’s how he designed his math program.
It’s common sense. Give the students plenty of chances to review material so they retain their skills.
Public schools continually experiment with new methods without bothering to study the research, if any, to see if it works. This is true of language arts, math, science, etc. Perfect way to dumb down the schools.
Experimenting on the kids by constantly changing the curriculum is a recipe for disaster.