Me and my best friend were on vastly different sides at the same time in our lives age-wise with similarly difficult babies. But one coped like it was all a breeze and one had crippling PND. These are the reasons I can identify where we differed:
1- different temperaments of husbands (as well as different ages of husbands). One had a significantly older husband who was quite highly strung, one had a younger husband who was pretty chill (maybe too chill sometimes!)
2- different family attitudes to mental health. One had a family with quite open MH issues who talked about stuff, one had a buttoned up family with no MH issues ever acknowledged or discussed.
3- different lengths of relationship. One had been married over 12 years and were an incredibly established couple, very used to their dynamic. One had been married a year and only known her husband 2.5 years, less established and with less defined rules.
4- different family support/kids in family already. Vastly differing situations here for us both.
5- different general dispositions. One quite nervy and already had established links with MH help, one very much never having had MH issues before.
Probably other factors, but those are the main ones. I wonder if you can guess which of us had PND and which didn't, as what I've detailed above doesn't entirely fall with the one you'd think would have trouble for all categories. Showing it's a combination of factors.