Simply....
Money
There are significant tax incentives for film production in Georgia (both movie & TV)
Time has an excellent article online
Georgia has been working to attract Hollywood since 2008, when then governor Sonny Perdue signed a generous tax incentive for film productions. Thomas says officials lobbied for the measure after the state lost the production of Ray, the biopic about Georgia native Ray Charles that won Jamie Foxx an Oscar, to Louisiana. Georgia now offers a 20% incentive on productions of $500,000 or more, plus an additional 10% if the film adds a peach logo to its credits. The incentives are similar to those in states like New York and Louisiana, but in Georgia, unlike those states, the law has no end date.
Source
In 2007, the film industry spent $93 million on productions in Georgia. In 2016, it spent over $2 billion. In the past decade, the tax perk has attracted the Hunger Games franchise, the Fast and Furious movies and superproducer Tyler Perry, who has made the state his base. Television hits like Stranger Things, critical darlings like Atlanta and reality series like The Real Housewives of Atlanta have all set up shop in the capital, often for years at a time. Georgia’s government estimates that in 2016 alone, the film industry gave the state a $7 billion economic boost through job creation and tourism.
...and co-operation
It takes more than tax incentives to lure productions. “It’s not enough to just get us here,” says Walking Dead producer Tom Luse, a Georgia native who began his career as a production manager on the Georgia-based film Glory. “The mayor [of Senoia] and state are eager to help, allowing us to shut down bridges and streets and take over towns.”