I think that, for as long as they think that it can be a quick fix, they prefer to keep everyone on board - because if they return to the terminal, let everyone off, and then try to round everyone up again, that all takes up valuable time and adds to the delay.

Of course, it's no fun sitting there for hours with little info, and especially without air, but they probably did not originally think it would take so long.

Is it the Mumbai flight?