Bullies typically choose a kid that somehow differs from the rest of the group. How specifically that kid differs, that depends on the specific group. In some groups, it will be the kid of a different race, in other groups in will be the nerdy kid. If there are multiple candidates, the bullies will probably choose the "safest" one, that is the one with smallest risk that someone will intervene and punish them.

The culture can have an effect on evaluation of the "safest" victim. If everyone knows that no one important will care about a black kid being bullied... And in a different town, maybe everyone knows that no one important will care about a kid from a poor family.