From the Background: "Family reunification and the admission of immigrants with needed skills are two of the major principles underlying U.S. immigration policy. As a result, current law weights the allocation of immigrant visas heavily toward individuals with close family in the United States and, to a lesser extent, toward individuals who meet particular employment needs. The diversity immigrant category was added to the Immigration and Nationality Act (INA) by the Immigration Act of 1990 (P.L. 101-649) to stimulate 'new seed' immigration (i.e., to foster new, more varied migration from other parts of the world). Diversity visas are allocated to natives of countries from which the combination of immediate relatives, family preference, and employment preference immigrant admissions were lower than a total of 50,000 over the preceding five years combined."
CRS Report for Congress, R45973
Congressional Research Service: https://crsreports.congress.gov/