Perhaps it's because they don't want "the Jetstar mentality polluting the Qantas culture"
Ok I am joking slightly as it's rumoured that that was what AJ said in the beginning of Jetstar, but it does has some truism to it.
Perhaps some of the applicants weren't what QF were looking for. Hell there are more than a few QF pilots Qantas they wish they didn't take.
Perhaps some applicants they deemed weren't suited to the time as being a SO. Especially since history shows Qantas usually hangs the SO out to dry in a incident.
Perhaps there were just other applicants higher up the waitlist especially considering the number that applied.
Either way, Qantas is a good job, but it's not the end of the world if you don't make it and there is no saying one won't in years to come. There has always been people who don't get in who we all believe should. The system is not perfect. The grass may be pretty green here right now, but the possibility of another drought? Well it is summer after all