Here is an extensive article with over a dozen links which makes the case for restricting Muslim immigration in Sweden. If a nation, or city refuses its first, basic responsibility, that of enforcing with violence public order, then Barbarians Will Come!
Its as simple as that.
They may have reasons. No, they will have reasons, but the best, truest reason is that it is more fun to be a barbarian than to be a low-status worker bee. So you have to retain the ability to smack barbarians on the nose, or you might as well surrender now.