A considerable portion of the US lives in the southeast. Georgia Tech is a strong CS program that is outputting a ton of qualified graduates. Not everybody living in the southeast wants to move to SF. The Southeast also has much better representation in traditionally underrepresented groups in CS, enabling tech companies to mitigate some of the "pipeline problem" that makes it basically impossible to achieve diversity goals. There are also few major tech companies with hubs in Atlanta, meaning that the first ones to really develop there can scoop up talent more easily.