I thought he did say why he thought hypothyroid people are becoming more prevalent. They don't die as children. He specifically targets therapies such as antibiotics and tuberculosis cures that save especially children who would otherwise die. He says they are susceptible to infection (i.e., weak), they grow up, and they reproduce their weakness. In his opinion, this weakness is due to being hypothyroid.
In the part I read last night, he says much, though not all, of the tendency to weak thyroids is passed on by the mother's mitochondrial DNA, which cannot produce enough energy to support a high-performing thyroid. (As I understand it, mitochondrial DNA is separate from the cell's nuclear DNA, which has contributions from both father and mother.) This lack of energy production, for instance, is one of the issues the Dr. Abraham's ATP CoFactors is meant to address.