Because they're poorer and there's less immigration. And if there's any, it's kept in check by the indigenous criminals rather than by the police.
Also, it's by perception (where it is most "felt"). I suppose one should also not be surprised that most petty crimes and theft are not even denounced to the police in the most "mafia-heavy" provinces, while in other parts of the country you go to the police even when you find a broken headlight on your bike.