Well, they didn't land at the wrong airport. They botched their take off procedures and missed the mistake in over multiple checks, ignored multiple warnings then fucked up even worse trying to fix it while in the air. The situation got so bad that they needed to land to fix it, and because Sydney had weather and they screwed up their instruments, they were diverted to Melbourne.
It is difficult to explain to lay people just how royally they fucked up, but almost literally a trained monkey could've done a better job. They were lucky they were in Australia, which has top notch ATC and literally saved their butts. If this happened in Malaysia or Indonesia, the outcome would've probably been headline news.