I've always wondered too. There are probably a lot of reasons and for this if it is in fact true (which to me in my observations it seems to be). I don't think it's specific to black neighborhoods as I've seen it in Asian and Hispanic nabes as well.

I think the "socio economic culture" you come from is the main reason. For example our neighbors (when I lived at home) were white and our neighborhood mostly was as well. However these neighbors were a little more cavalier about the condition of their houses exterior and interior. They in fact go the whole neighborhood on that "store all your shit in your garage and park on the street" thing. Anyway many would call them "white trash" or at the very least have "white trash" tendencies. They were god enough folks but yeah-- one day they just had a whole couch out in the front area to be picked another time it was a toilet.

tl;dr if you're from a culture that lacks of upkeep you're likely to perpetuate it.