Why was slave labor so important to the economy of the southern colonies?

Why were chinese american immigrants frequently detained when trying to immigrate through angel island? the rules for immigration were stricter at this checkpoint due to health issues that were prevalent among asian populations at this time. the location of the island was difficult to reach, and ships to the mainland were often prevented from sailing more than once a week. immigration agents were notorious for asking immigrants confusing questions as a means of denying them entry. low wages caused a shortage of immigration officials, so many immigrants had to wait weeks or months before their applications were processed.