People's perceptions of risk and the remaining COVID restrictions are what continue to affect NBL crowds.
e.g., AEC requires full vaccination for entry; people with COVID or close contacts thereof have to isolate and can't go to games; etc. Based on current new case rates and vaccination rates, that'd mean 10%-15% of the Adelaide population wouldn't have been able to get into Adelaide's home game last night. So I'd expect those restrictions alone, with current case rates, to be knocking AEC home crowds down by 900-1,400 people. That's before you consider how many people would rather not go to crowded places or won't go until they can attend without a mask.
On a related note, a local sporting club I'm involved with has kids who have yet to come to a single training session for the year. This club has been training 3 times a week since school started - 7 weeks! These kids have been excluded as close contacts for all of that time, due to multiple COVID positives at their schools or within their families. So the close contact isolation rules have had a huge impact on families with school kids in particular. That must be hurting 36ers' home crowds.
Until restrictions have gone altogether, don't expect crowds to recover to 2019 levels.