If there were ability grouping and teacher specialization in elementary school as in high school, the parents of the children in the bottom group might be unhappy. The heterogenous model is easier politically for principals and conforms to their philosophy, so it is entrenched.
Not necessarily. I know parents of slower learners who are unhappy for the same reason that people here are: the pace is wrong for their kids and their kids don't learn. Recall the debate in the 70s and 80s, when parents of disabled kids and slower learners got very loud about their dissatisfaction with the schools. They were unhappy that the needs of their kids were being ignored.
My cynical opinion is that lockstep teaching is easier for teachers and administrators (even when enhanced by IEPs). It probably also reflects the romantic idea that everyone can learn at the same pace if we just give them a chance.
