I weaned my first DS at 4 months. Though I will say we didn't exactly have a "successful" start as you mentioned with these women. If I had to name a reason I switched to formula feeding, I guess I would say just complete lack of knowledge. My DS was 4 lb 11 oz and a very small, tired little guy. His latch was lazy, but I didn't understand anything about what that meant at the time. Because I was afraid he wasn't getting enough, I pumped and fed, OFTEN. Sometimes in place of nursing sessions, hence lessening his nursing abilities. I had to return to work at 6 weeks which lead to MORE pump-and-feeds instead of nursing. I wasn't pumping while I was at work (again - serious lack of knowledge here)... I didn't really know I was allowed, much less entitled, and I wasn't prepared at all to do it. I was very young and overwhelmed and just essentially clueless. The fewer nursing and pumping sessions of course lead to low supply, which lead to formula supplements & eventually weaning. I was also on WIC at that time, and they seemed to think weaning was a good idea. I was very mainstream at the time, I thought my experience was pretty typical (in fact, I breastfed longer than any other mom I know IRL to this day).
Now that I have MDC & have found myself to be an AP parent, I do things much differently. I knew when I started TTC DD last year that I would find a way to breastfeed successfully and do it for the long haul. I got an LC before she was born and educated myself about typical problems and solutions, what to do to improve hard situations, how to keep my supply when I returned to work & etc. DD will be 4 mo in a few days, so I've made it as far with her as I did with DS and haven't supplemented in anyway. I actually plan for CLW with her.
Also, I know I had a much different thought pattern when I was a more mainstream individual. Breastfeeding was HARD and EXHAUSTING and I just didn't really see why in the world I would kill myself like that when I didn't have to.... I genuinely believed that formula was just about as good as breastmilk (Have you read a formula ad lately? They pretty much claim this to be true). I did not sleep train DS, but I do remember thinking STTN was a really important milestone or something like that. My DD actually DOES sleep through the night & it kind of makes me sad... like I'd like more time with her - because my point-of-view has completely changed. I'm just saying, among the zillions of reasons women may choose formula, complete ignorance IS a true possibility. When I had my first DS I didn't know there were women in the US that breastfed past 6 months; literally.
Also, as a PP mentioned, it's hard to be a formula feeder around anyone pro-breastfeeding. I still really regret not nursing my DS his due time. But I didn't know what I didn't know... and all I can do now is feed him whole foods & breastfeed DD & any future children. But I still feel the sting when I have to say "DS was formula fed". Don't be too tough on them.