Ok, I will explain what I have heard from so many stranded Aussies.
This announcement from the prime minister in March 2020 was made to bring Aussies back within a 5 week time frame. This was only applicable to those who were on holiday, really. But not everyone was able to or willing to due to a number of valid reasons.
What if it was your daughter who stayed because she was partway through her study in the UK? What if it was your brother who couldn't get a plane ticket because they were all snapped up within days of the announcement? What if it was your sister who couldn't sell her car, escape her rental agreement, or exit her work contract in time? What if it was your dad who flew with his disabled elderly mother and couldn't return? Or your best friend who was on their way home but the flight was cancelled and they got stuck in some random city.
These people had lives overseas. Obligations that they couldn't just drop when the virus was only just starting to get bad. These people shouldn't be ostracised from the country for making a decision with the information and situation they were in.
That is why they didn't come home when the call was made.