Why was Ogelthorpe's prohibition of slavery reversed?
Primarly driver was profit, but it did help that Georgia was a neighbour of slave state that profited hadsomelly from the slavery. So Georgia jumped in. And lastly timing Oglethorpe died thus there was no reason to retain the prohibition of slavery policy.
The colony of Georgia was started in the 1730s and originally prohibited slave holding, but that prohibition was dropped after some years. Being next-door to a slave holding colony (South Carolina) seems to have doomed the experiment in having a free-labor-only colony.
Georgia did not “originally prohibit slave holding”. That prohibition was introduced in 1735, years after the state's foundation, and was not based in morality but in Oglethorpe's belief that the presence of negro slave labor weakened the white establishment. Instead of Negroes, indentured white labor was imported. The law included the non-humanitarian specification that Negro runaways be returned to other states.
Several years later, around the late 1740's, the slave restriction was reversed due to the profits from rice agriculture. By 1776, the Negro population exceeded the headcount of the whites.