The reason districts in South Carolina's lowcountry were smaller compared to those in the backcountry is primarily because the lowcountry had a higher population than the backcountry. This higher population density necessitated smaller districts to ensure proper representation. As a result, the lowcountry, with its more populous areas, had more districts compared to the backcountry, which had a lower population density. Therefore, the correct answer is:
The lowcountry had a higher population than the backcountry.