It's due to the quarentine rules I read. I can't see Australia going ahead for the same reason but I guess things could change by then as it's not until November.
I was convinced after the disaster last year with the last minute cancellation that they will never get back to Australia, but they put massive money in rebuilding the track for this year that I get the feeling that they may be willing to not insist on a whole two weeks quarantine, especially if most of them are vaccinated by then anyway. They already didnt insisted on a full quarantine for the Australian Open earlier this year (for those players that werent on a flight with infected people), instead they werent allowed to move out of their bubble (meaning outside of training venues, gyms, etc.), I reckon the same could be done for F1 extending to the race track (Budapest did this last year for all UK citizens which is basically the whole paddock). And even if not the races before Melbourne are all a bit in danger anyway, so even a two weeks quarantine might not kick the race out.
Japan is to me the race that is unlikely to get ahead. They are similar strict on this and F1 is pretty irrelevant there (although might be different if the Tsunoda and Honda hype properly kicks in), so you cant expect them to turn a blind eye. Mexico is also very unlikely because the track is currently used as a temporary hospital, not convinced that this changes by then.
I don't particulary mind though, they likely have many replacement venues to insert in (just like Turkey will likely replace Montreal). For Montreal in particular the issue is just that there are already rumours that this cancellation may cost them the slot on the calendar for the long term future, which would be a great shame.