Sweden quite simply took way more poor migrants (many of them for humanitarian reasons) from developing world than it could effectively integrate, and that failed integration resulted in hopelessness, high crime and unemployment in these communities. Most of them ended up living in cheap apartment blocks located in suburbs while more well-off people started avoiding these areas, and that's why those suburbs are the bad areas.

Obviously you also have also suburbs with villas where richer people live and little crime. Sweden at this point is a highly segregated society.