Oscar Newman's theory on "defensible space" explains this.

If a person lives in a large shared area, they don't identify with it and as a result don't respect it (e.g. tower blocks, council estates). They don't have a "defensible space" except their own house/room. Why should they have to look after something they don't actually own? That's why graffiti and litter is quite common.

People in 'nicer' neighbourhoods have cleaner neighbourhoods because the house and the land attached to it is essentially their own and no one else's. They take pride in it and look after it, as do their neighbours.

Just thought it might be an interesting thing to share.