The origin of the South Carolina Regulator Movement can be attributed to a combination of social, political, and economic factors, particularly the first option you mentioned: "Very few courts and no sheriffs in the backcountry led to lawlessness."
In the 1760s, the backcountry of South Carolina was experiencing significant lawlessness and disorder. The distance from the established low-country settlements, where most of the political and economic power was concentrated, created a vacuum of authority in the backcountry. With very few courts and law enforcement officials (sheriffs), residents were often left to fend for themselves against crime and violence, including theft and aggression from neighboring settlers and outlaws.
This lack of representation and protection in the backcountry fueled resentment toward the colonial government and ultimately led settlers to form their own regulatory bodies to bring order and seek justice. The Regulator Movement sought to establish local control and enforce law and order in the region, highlighting the social and political tensions between the rural backcountry and the more established low-country areas.
While other factors such as economic disparities and fears regarding land may have played a role, the pressing issue of lawlessness and inadequate governance was the primary catalyst for the emergence of the Regulator Movement.