What's missing in a lot of the "explanations" here is a very important second half of the sentence.
I'm reading a lot of "there was no way to let the passengers off," when I think the actual truth was "there was no way to let the passengers off without inconveniencing the airline or the airport staff."
There is always a way to let the passengers off. 738s have a built-in front stairway. Mobile stairways abound at every airport. No gate? No problem; there's always some door somewhere leading from the ramp into the terminal; passengers could be deplaned and directed to that door.
People get so hung up on "this is the standard way we do things" that they fail to take a step back and realize that there are alternative solutions available.