Probably part of it. Also, business leaders on CofCs who complained about graduates not being ready for the workforce. This lead to schools being forced to adopt a business model of education, where only the bottom line matters. This is just a modern version of the factory model school.
To
@BradStevens, who used the word "comfortable", I would call that a gross oversimplification. Kids should feel the joy of learning new things, not satisfaction with meeting the quarterly projections. When kids enter kindergarten, they are eager and energetic. From what I understand, just their natural curiosity is enough to carry them to pretty impressive growth levels during the year, regardless of the school setting. But by the time they reach 3rd grade, their joy of discovery, eagerness to learn, and passion for knowledge has been beaten out of them. By 3rd grade, they are clock punchers and paper pushers. Everything is governed by a top-down pacing guide, which allows for learning that is miles wide, but about an inch deep (depth would take time for discovery, btw) with little time for serendipitous learning opportunities. The pacing guide, of course, is based on state standards, which are measured by standardized tests produced by multi-billion dollar companies.
I'm surprised schools do as well as they do.