Miami is interesting because of Lebron, but they still struggle with fans. Miami isn't a sports town. I'm not sure why, but I wouldn't be surprised if it was that there's so much going on there.
I'm not going to argue that Atlanta isn't a bad sports town because attendance is poor across all non-football sports. But it's not because they are naturally sports stupid or something. It's because of accessibility and transplants. And when attendance isn't there, franchises won't commit to the spending that facilitates a growing fan base.
Turner field couldn't be in a worse spot. If it was located in North Atlanta (perhaps near the Lindbergh stop), they wouldn't have an attendance problem.