I believe one of the reasons why they're proposing it in south Forsyth is because there's very little there compared to the rest of metro Atlanta, while the area is also growing quickly. They can build up the area around it, build for future infrastructure improvements, etc.
But the problem with building closer to Atlanta is... where ya gonna put it? Will landowners sell their property in Doraville or Smyrna? If so, what about rezoning the land for a mixed-use development? As much as we might love to do it, we can't just open Cities Skylines, destroy what's on a parcel of land and drop an arena on it, so I don't think it's possible to put a mixed-use in any of those areas, much less just an arena. I'd like to think these developers investigated that possibility before choosing south Forsyth, but I don't have that answer.
You're not necessarily wrong about the area... but transplants are *everywhere* in metro Atlanta. Including me. But there is absolutely an identity for a potential franchose to adopt, NHL history for it to embrace (the former Atlanta/Calgary Flames living in the area were never embraced by Turner or Atlanta Spirit). There's plenty that an organization can do to adopt a local identity, even from 30 miles away.
South Forsyth isn't ideal, in my opinion, but it apparently fits the vision the developers have.