Paraphrasing
'it is an unstable culture held together until it loses economic stability.'-I think, though he does not outright say it, the crux of his book, and the problem he discusses.
I think what happens is his people were ok when they were in the holler in Jackson Ky, and still ok when they had jobs in Middletown Oh. Once they lost their jobs their life went to shambles, as is well understood by social scientist and explained in the book. They(his grandparents and ancestors) were ok in the holler because they had one another, and a social structure(usually headed by a patriarch or matriarch) that kept everyone busy and feed. But if they were in Middletown for 20 or 30 years, the family structure in the holler back in Ky had crumbled or just changed so much that it was no longer available for someone to 'come back' to. His mamaw, papaw, mom, his sister, and himself could no longer return without buying their own land, house, etc. And, even if they did return I doubt the other uncles and their kids were their either; just some distant cousins, they barely remembered.
Perhaps a few mistakes, simply become compounded and over decades create a greater problem. His mamaw gets preganant at 13; Mamaw and Papaw flee to Oh as a teenage couple, and fight for the next 20 years stuck in teenage maturity; their kids suffer these consequences; his mother, though intelligent and a good student, gets pregant at 19, then divorced; begins her slow slide to destruction; thereafter JD's childhood.
Not saying people cannot get married and have kids young while being successful; they do. But it seems, when that happens in conjunction with moving from their family structure they have known their whole lives, only spells disaster.
Just as JD needed a series of good events to help him get to where he is, so a series of bad ones lead to his childhood being so hard.