Georgia has a long tradition of "local rule" in which the State delegates a significant amount of decision-making power to local governments. Aside from the 536 officially chartered cities, Georgia does not have "townships", "towns", "villages", "boroughs", or other significant administrative divisions at the local level, so county government fulfills all of these roles. This means that the local county government has significant local power over the day-to-day affairs of its citizens: taxes, police and fire services, courts, public health and safety, water & public works, roads, and zoning are all handled by the elected county government. While education and sheriff/jail services are technically separated under independently elected entities (the county Board of Education and Sheriff, respectively) and not appointed by the county commissioners, in practice these officials are beholden to the county government due to the latter's control over taxation rates and allocations.

In short, being a county commissioner in Georgia carries considerable political power and these positions were usually held by a powerful local politician and his close associates - think Boss Hogg in the Dukes of Hazzard. During the reconstruction era many areas of Georgia prospered and grew in population. Ambitious local politicians who were not part of the existing county's "good ol' boys club" were constantly petitioning the state legislature to break off their local area to form a new county or city. The legislature was fairly receptive to this and approved dozens of new counties and hundreds of new city charters at this time.

Ed Jackson, at the University of Georgia, has written a historical oversight of the growth of the Georgia counties, available here.

Follow up question. Why are some counties in Virginia county-cities? As in the county encloses the entirety of the city and not much else.