What outside factor most directly contributed to the formation of enclaves in cities?

The most direct factor contributing to the formation of enclaves in cities is migration, particularly the influx of immigrant populations seeking economic opportunities, safety, or community. These groups often settle in specific areas, creating distinct neighborhoods characterized by shared cultural, linguistic, and social ties. Economic factors, such as affordable housing and job availability, also play a significant role in attracting these populations to particular locales, further solidifying the enclave's existence.