It s partially the parents and partially the academic institutions.
Many of the private elementary schools added a grade option between preschool and K to simply expand their “market base.”
It is sold as a way to allow your a child to mature academically, socially and athletically. And many parents see the benefits and do it.
But it’s mostly just a way to get parents who are willing, to pay tuition for one more year.
Many high schools that have middle schools recommend kids who transfer in from public schools during 8-9th grade to repeat their previous grade to get up to speed. The reality is it’s more so a way to ensure their consumer attends their school for 4-5 years, not just 3-4. Another year of a paying customer. | 
