I think a lot of those cities with triple the number of breweries by population-to-breweries compared to Austin also have a much great population density surrounding those breweries. I feel like the ease of public transportation obviously plays a big role as well.
Austin isn't that difficult to get around, but the closer the breweries are to me and the closer they are to each other, the more likely I am to visit them with frequency. I've been to Zilker and Lazarus more than I have Live Oak's new location, not because I think they're higher quality than LO, but because they're close to my house, close to each other, and close to other places I frequent.
Assuming breweries in and around Austin grow in pockets, like they have with Burnet/Metric, East of downtown, and even out on Fitzhugh, then I do think Austin has some continued room to grow. And I do agree with you that first and foremost, a brewery better produce quality beer. However, with trips to the breweries being such a communal thing that people often do in groups, making it easy and fun for people to plan trips and bounce around from place to place is almost as important, if not more so, than the quality of the brewery for a lot of folks, IMO.