It has not yet been mentioned in this thread... and obviously none of us know for sure. While it's true that they could have done nearly anything they wanted, we would do well to remember that there is always the possibility of unintended consequences of keeping the flight open the additional 10 minutes needed to print boarding passes and update numbers. I don't know what those possible unintended consequences are, but they could include flight crew timeout, slot/gate issues, weather issues, overtime costs, curfew penalties, etc. Some of those unintended consequences would only affect WN's bottom line, but some of them could affect the 135+ pax already on the plane.

In other words, the choice could quite possibly have been: A) reopen the flight and let these passengers on, at which point the flight crew has timed out and we have to offload the entire flight and wait for a new flight crew... or B) hold the line and keep these passengers grounded so this flight can take off.