In my mind it all boils down to a matter of: 1) what is the quality of the public school option; and 2) if the public school option is not strong, can the parents afford private school.
For example many parents in NYC are not satisfied with the public schools for various reasons and have the resources to send their kids to private schools where they feel their kids will get a better education. In contrast others live in places with excellent public schools and choose to go the public school route through HS even if private schools are affordable.