It seems as though street gangs in the early 1900s were largely comprised of Irish, Italian, Polish etc. immigrants or their descendants. Now it seems as though street gangs are mostly comprised of African Americans and Latinos. When and why did this switch happen?
Gangs are inherently tied to poverty, at least, in their inception. Those in the lowest brackets of a society are not always guaranteed protection by the law. As such they turn to each other and create their own groups for protection and solidarity. In the time you described, the urban poor consisted of European immigrants, today urban poor is a mix of many races, though latinos and blacks do not hold the monopoly on street gangs. Aryan Nation, white motorcycle clubs, and the Russian, Irish, and Sicilian mobs all exist in various capacities.