Probably because elite schools have smaller class sizes or teacher/student ratios thereby making it less necessary to have the ability to disseminate information via mass means with technology. Put it all up on a big screen where 30 kids can see it, send the assignments out to 120 kids via google classroom on school issued chromebooks (because there are plenty of kids from families that cannot afford computers), and do all the grading and review digitally. I’d be willing to bet those expensive private schools use plenty of tech, maybe kids carry Macbook Airs instead, but there’s no escape from tech in schools.