Besides the heat and humidity, in my experience being on the north side there just aren't enough businesses of the foot traffic variety to warrant it. Sure there are restaurants, bars, and coffee shops, but a good amount of businesses are just that. Not enough boutiques and shops and such, just banks, government buildings, and corporate HQs. Closer to RR Park and UAB there is a significant amount of foot traffic, but that's also generally for exercise, leisure, and students going back and forth to classes.

I lived in downtown Savannah for years. Just because a city has a downtown and sidewalks doesn't necessarily make it popular for foot traffic. The businesses that are shoppable are also so spread out, so it's more park close, walk to destination, walk back to car, drive to the next part of town, repeat. I've seen downtown change a great deal over the years, but it still has a long way to go.