Can remember 1 snow game in raiders history, man up and be realistic
And I can remember dozens of them where the temperature was roughly -5°c with windchill, conditions that only a relative handful of Sydney fans would tolerate. Most games in winter don't get quite that bad, but they don't need to for most people over roughly 60, anyone with young kids, or various medical conditions, to call it quits because it's too cold.
Put simply there're only so many people whom are willing to pay to sit in such conditions over staying in the comfort of their home and watching for free.
In Canberra's case you can scratch between roughly 3-5k off the attendance each time it gets so cold, and you wouldn't even need an enclosed stadium to fix that problem either, though to be clear that would be the ideal outcome.
Coverage over the seats, wind breaks, and some large outdoor heaters built into the stands would be enough to make a significant difference, but if you're going to go that far you may as well go all the way and just enclose the stadium.