Melbourne and Sydney (and Adelaide) have historically always had mild weather. The extremes we are starting to see now had only been occasional. Now they are consistent.
energy prices have always been deflated. There has always been plenty of cheap natural gas avalable.
commercial builders have been reluctant to embrace new technologies like aerated concrete panels and the market does not support highly rated insulation batts.
inflated qualified labour pricing has resulted in builders and improvement contractors taking the quickest, simplest option instead of the best option (eg. blow-in powdered insulation instead of batts).
I think it is mainly because;