Because the vast majority of people support a club not the sport/league.
Most Aussie Rules fans in Canberra have little interest in GWS matches unless the team they support is participating. You can participate in performative stupidity and pretend that isn't the case if you like, but it is what it is.
Regional Tam's sample for Southern NSW is 635 Households.
230 of those are from Canberra, 227 from Wollongong, and 179 from the rest of Southern NSW as they define it (Wagga, Dubbo, Orange, etc), which is different from how Southern NSW is traditionally defined.
I don't know how I can make this any clearer for you, when you see ratings data claiming to be just for Canberra it's extrapolated from the Southern NSW data of which Canberra/the ACT represents a minority of the sample, and as such it's influenced by the rest of the Southern NSW market which is an extremely different from the ACT both culturally and in their social make up. In other words they're literally glorified guesstimates informed by poorly sampled data.
Again you're arguing against a claim I never made. . .
I never claimed the AFL or AR is or would be bigger, I said that Aussie Rules and RL are roughly the same size in Canberra, i.e. neither is particularly more popular than the other, and that a Canberra AFL side lunched tomorrow would be as big as, if not slightly bigger than the Raiders, from day one.
The first claim is simply true, you might not want to accept it but you're simply ignorant and it is what it is, and it isn't hard to imagine that the second is likely given the different attitudes towards attendance and memberships between the two sports and the pattern of AFL sides converting more of their fans into members and attendees on average than their NRL counterparts.