I think this - and the fact that most of their J cabin seems to be filled by free upgrades - is a symptom of a broader problem for Rex - they have done nothing to lure business travellers.
The vast majority of people choosing a 6-7am flight are business travellers. Why would they choose Rex over QF or VA, the latter of which has fares for the same price that also include seat selection, access to
decent airport lounges and all the benefits that come with having a frequent flyer program. Rex doesn't even have a frequent flyer program yet, which frankly in Australia is a must-have if you are trying to compete for business travellers.
The express PCR test at the SYD int'l terminal is now $79 with a 90-minute turnaround time