Kids who attend Montessori schools are probably a self selected group of high achievers - not a random sample of kids, or a group of all kids born in a certain year and living in a particular school district. I know some kids who are just plain not very bright, some who are average, and some who are smarter than I'll ever be. Public schools don't have the luxury of booting out kids who don't measure up, nor can they raise tuition as needed.
I agree that all kids can learn, and probably nearly everyone can learn algebra, but not all at the same pace and the same timetable. Is it really critical that kids learn this stuff in 6th grade? What are they supposed to learn before finishing high school that I missed out on?