A few years back, I was on a QF B737 which did an emergency landing into NTL. The pilots tried to land in SYD twice, failed, circled, ran out of fuel and declared an emergency... then instead of circling around for a 3rd go at SYD, they flew straight into NTL, with fire engines screaming around us when we eventual made our uneventful landing.
Anyway, aside from all the other issues, once on the ground we all discovered that Qantas doesn't have any stairs which are suitable to connect to a B737! After all it is a Qantaslink port. So the poor Qantaslink ground staff ran around looking for some big stairs, and decided to grab one off Jetstar.
Well... A320s are a sightly different height to a B737, so their fixed stairs don't quite line up correctly with the doors on a B737. Although they were close enough for the staff to hop over the few centimetres and discuss the situation with the crew, this gap presented too much of an OH&S style issue for passengers.
It was quite late at night by now, and all the Virgin staff had gone home, but somehow, someone managed to swipe a set of VA B737 stairs and we all got off!
Moral of the story, there are possibly all sorts of logistical reasons for why Qantas does what it does beyond our scope as mere passengers.