More cost effective and answered to the business serving travelers. TSA answers to nobody and customer service is not even a priority. Private security that harassed, berated, and abused patrons would not be working private security for very long.

You will have to define effective to tell if TSA or private security is better. TSA is an effective hinderance on travel, jobs program, and public money funnel for scanner companies. Private security was more effective at scaling security for the risk and trying to be a minimal hinderance to travelers.