This is just a guess, but I think-hmmm-yes, I think it may have something to do with hormones, actually.
Is it any surprise, too, that both of the people they have chosen to quote are both men?
Here are some snippets:
Fathers usually feel elation after a birth, Coleman said, but that feeling of "engrossment" can fade away, depending on family circumstances.
That can happen "if the mother is very, very controlling and wants the baby all to herself," Coleman said. "Also, fathers can experience frustration, sexual and emotional, if they forget to remember that the wife is not interested in sex at that time. If the wife is very motherly and maternal, he might feel kind of useless, on the periphery."
Depression in a father leads to a well-known pattern of behavior, Coleman said. "He tends to work longer, to watch sports more, to drink more and be solitary," he said.
If they forget to remember?
Alright, I don't doubt men feel alone, neglected, ostracized, ignored, and -dare I say it-sexually starved.
The attention has shifted. The roles of Motherhood and Fatherhood are, indeed, different and difficult at times. In some cultures, the father isn't even involved until after the child is weaned.
I also don't doubt that the father feels and experiences some sort of depression after a baby is born. It is a huge change. But I wouldn't call it postpartum depression.