1) They were Georgia rappers that weren't from Atlanta and never claimed to be. Nobody has blown up without claiming Atlanta even if they weren't from there. You might say Bubba Sparxxx, who had a gold album and single, but think about it, he had prime Timbaland production.
2) Being country at a time when country was still frowned upon. They were for more country than Kentucky's Nappy Roots, who went platinum. Field Mob sounded overtly and unapologetically country even to Southerners.
3) Great music in the wrong era. They dropped "Project Dreamz" at a time when bling was huge. Complete opposite of what was popular at the time.
4) No Follow up. "Sick of Being Lonely" and "So What" were big songs, but they didn't have a follow up to build on the momentum.
I could see them re-branding themselves like 2Chainz did last decade during that same era where 2Chainz blew up. By that time though, it looked like they were disgruntled with the industry and with each other.
Jazze Pha gets a hard time on this board, but some of his best work is with Field Mob from "All I Know" to "Sick of Being Lonely" to "So What". Maybe them signing to Jazze's Sho Nuff if for nothing else, but having him helm or entirely produce an album for them may have helped.