First, these are very exploratory numbers, not to be taken as the last word on the subject. There does appear to be a disparity, though. I think it comes from having different goals. Many parents want high academic achievement for their own children and may not care about the culture of the school if their own children are doing well. Some middle school students may chafe under the kind of supervision that comes with close personal connections between teachers, parents, and them.
Teachers see the payoff in improved classroom management and many other areas, so they love it.
Your book states that these strategies aren't new. My guess, and it is only a guess, is that resistance comes from a lack of understanding and parental anxiety about anything that might possibly interfere with their children's competitive advantage. Parents are so worried about their children's future these days that anything that departs from the traditional may seem dangerous. We are so caught up in the corporate, factory-style educational process that alternatives seem less rigorous and therefore less desirable. The evidence in favor of small schools is compelling.