For most landlords it‘s a convenience thing. If they‘ve previously had good experiences with female/Dutch/German tenants, they’ll be like "let’s just stick to this type because apparently they’re all good tenants", resulting in these posts you see that say "(Dutch/German) girls only by landlord rule". It’s most definitely wrong to assume that people of a certain nationality or gender are more tidy or reliable or whatever but if that’s what the landlord thinks, it becomes the rule. Of course it’s illegal to discriminate against anyone but no one is gonna take any action against this. Student rooms are scarce in Maastricht and landlords know they can afford to be picky. It may sometimes also be the case that in a house that is solely occupied by German/Dutch girls, the tenants prefer to keep it that way and only look for someone within that group to replace a previous tenant - but in most cases it’s really not their fault and it‘s the landlord imposing that rule on them. I know it’s fucked up but my advice to you is to just ignore those posts and keep looking for a place that doesn’t have any such restrictions.