The same reason you don't say black beggars or Mexican criminals. The racial or national background of these people might be interesting for sociologists but it's unhelpful in everyday discussions. It strips people of their dignity, whether it's intentional or not. Don't be that guy.
It is interesting and helpful, because the reasons that these beggars are operating in Sweden was given (internal migration within the EU).
If he hadn't said it, replies would have expressed surprise that the prosperous Swedes had resorted to aggressive organised criminal behaviour, and there would have been speculation on why that is.
There would also have been questions on why the government hasn't done anything about it yet, which he also addressed in his comment.
Look, this is about good manners, not censorship. Everything you've just said is trivial and unhelpful. Naming ethnicities this way will only make you look like a fool and undermine the point you're trying to make.
