It should be noted that both Frankfurt and Stuttgart are part of large metropolitan regions themselves, numbering more than 5 million each. These two are essentially connected by the 2.4 million Rhine-Neckar metropolitan region (they're the three essentially connected dots in southwest Germany, almost exactly in the center of this nighttime map). While I guess both Savannah and Augusta metropolitan areas would be able to support good small-medium zoos, direct comparison with Frankfurt or Stuttgart, situated in one of the most densely populated regions of central Europe, is likely to mislead.
Regardless, the nominal GDP per capita in Georgia is between that of France and the UK. There should be widespread buying power in Georgia, even though their Gini coefficient is high (higher than in any West European country). You'd expect more zoos in Georgia based on that.
I'd actually be really interested to know how many zoo visitors in this zoo-packed area of Europe are going to multiple different zoos in year, or whether they mostly stick to the nearest. I did notice there is a joint 'membership' arrangement in the Ruhr now, suggesting a fair number do visit more than one.