Because it works. Some reasons for this include: 1) Individualized instruction, based on the student's strengths, needs, and interests. That means that the child who understands can move on, and not wait for the stragglers to catch up. And a slower child can take the time needed to truly understand, rather than be pushed along when he isn't ready. 2) Parental involvement 3) One-on-one instruction 4) Students are more independent learners 5) Students are more well-rounded socially, because they are not stuck with only others of their own age group all day long.
Because the education system is going to hell in a handbasket and it's the only real way to become educated!