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You raise some very important points here. I had once also speculated to myself that a certain cultural malaise in America might come from the idea that the 'prototypical' White American will become a minority. I wondered a few times what will happen when this actually becomes true.
I'm more into thinking the explanation is class rather than stereotypes about bad Black parents. I think it's also the aesthetics of the pretty-looking missing girls; other missing children will simply have a less heartbreaking impact on viewers.