Why are people backing this untested guy who blew a winnable race over an accomplished two-term mayor with experience on the ground who knows how to get things done?
Statewide Democratic primary contests outside the ATL metro tend to be low-information affairs (and in the metro, only somewhat less so, depending on the amount of money being spent on media). The fact that a presidential primary (which tends to have much higher turnout) will coincide with the state/local primary this year will only amplify that effect. I expect considerable volatility in this race - and polls have never been great at pegging Democratic primary preferences (see above).
I suspect ballot ordering will also play an outsized role here. More people than you think will either be coming out for the presidential contest alone or will be turned out by various local primary campaigns and orgs. At the top of the ballot? Amico. At the bottom? Tomlinson.
