My personal opinion is the TSA is not developed, trained, or incentivized well enough to do the job properly.
They are not going to be able to stop any true threat. They are not trained or equipped to handle a active terrorist threat that has guns or bombs.
They are there as a 'show of force' deterrent, but to any well trained 'terrorist' they would not be perceived as a threat, in fact, a terrorist would be more concerned with the 'civilians' as you never know who may be a armed federal agent or off duty cop, or similar type of threat.
The TSA is there more to plant the message into the minds of travelers to follow the rules, don't carry weapons, or drugs, or drinks for that matter into an airport AND to make it appear as if there is a force present that can protect the people and the airport from attack.
Private security companies have more flexibility in firing poor performers, any agency the Federal government is responsible for running has the worst hire-fire rules and procedures imaginable in place, often over time the best employees will leave for better opportunity, while those that should be released remain, comfortably protected despite poor productivity.
Airports that have switched to private firms say they consider the contractors more responsive and better able to adjust staffing to address traffic surges and lulls. In addition private companies can tailor their training to include things a Airport may want, that the TSA does not train in, or does not agree to do.
And private companies can reward productivity and performance far better than the government can, I think everyone is aware that a government job is far better for those who lack motivation, and just want to show up and collect their paycheck than private firms.