That's a highly simplistic way of looking at it.
The vast majority of the first people to immigrate to Canberra were public servants from Melbourne. Obviously they brought their culture and Aussie Rules with them and that deeply influenced Canberra's early sporting history, however by the end of WWII Canberra had a well developed and highly competitive sports market where AFL, RU, RL, and Soccer were all popular in the community.
In other words yes Aussie Rules was more popular than RL in Canberra's early days, but no sport really owned the market like Aussie Rules owns Melbourne or RL Brisbane. Which aside from short periods in the 90s and 00s, when RL and RU respectively did dominate the market, very much continues to be the case to this day (though like in the rest of the country RU has died on it's arse in the last decade).
Currently Aussie Rules is probably the most popular sport in Canberra by a small margin, but that margin is growing steadily, and having our own AFL team would give the sport a significant boost not only locally but in most of Southern NSW as well. However that state of affairs would be easily reversible if the NRL actually gave a f**k, which they don't, so yeah...
Another thing that the code wars sorts won't tell you is that a truly unique Canberran culture, and the Canberran identity, didn't really come into being until the 90s and RL and the Raiders were deeply entwined in that. People didn't really refer to themselves as 'Canberrans' until the Raiders won the Winfield Cup in 89.