A barbarian is defined as an individual lacking refinement, learning, or literary culture; one from a culture that has not progressed to the stage of civilization. Due to the nature of socioeconomic and technological progression on Doa'Dane, nomadic peoples have become more or less extinct. With that said the Barbarian class is restricted to the half-orc race. Regardless of contact with neighboring human civilizations, the orcs preserve a simplistic, tribal society, and thus one of Orcish background would naturally come in the form of a barbarian. The only tribal humans on Doa'Dane are the Amentans, and their society is strictly isolated, thus one may not choose to hail from there.
This restriction can be overturned in the event a campaign setting predates the collapse of the Enojan Empire.