I think it is mainly because;

• 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).