From everything I've been able to find this isn't a repeat offender situation, they were tried and sentenced under Georgia's Street Gang Terrorism and Prevention Act since they were associated with a Confederate Flag "supporter" group called "Respect the Flag." These charges came shortly after the Charleston church massacre which may have influenced the severity of the punishment.
Basically a first offense resulting in maximum punishment due to the timing of the offense.
Idk, 15 and 20 years still seems excessive to me for a first offense. They are shitty people and so are the people they associated with, but if you apply this to every other shitty people criminal situation we'd have an even worse US incarceration issue than we do currently.