The slum of District 9 may hold aliens, but it's based on the Cape Flats slum erected during the South African apartheid era. So while it's occupied by non-human lifeforms, the slum aspect of it is depicted in familiar terms. You don't have unrecognizable alien trash; you have rotting box spring mattresses and discarded beer cans and flaming tires shooting soot into the sky.
It's easy enough to look down on the inhuman aliens as is. Forcing them to live in sub-human standards only makes it easier. Luckily, this slum is ultimately the arena of a much needed human ass-whooping that is a ton of fun to watch. You can't be too depressed in a movie where a dude gets shot with a pig.
