But can someone please explain why, in instances such as this, they just don't disembark the pax back into the sterile area?
US flights have requirements for additional screening at the departure gate. This consists of pat downs and additional bag searches and requires a whole bunch of additional security staff- probably 20 of them.
In terms of time you're looking at 15-25 minutes to unload 450 punters and their bags. To load them all back on board is probably 30 minutes after they've been through the security check. Add another 20 minutes for the security checks and you're looking at long turn around time to get them off and on again.