'didn't work out' is kind of short hand for, i'm not bfing and i don't want to talk about the whys and wherefors.
in my mind it can be any of a thousand reasons.
... maybe they never really wanted to, never saw anyone else do it in real life, had pressure or assumptions from family and friends that formula feeding is easier and or more acceptable.
...maybe they had a crap midwife who offered no support, or a midwife who was also trying to help 15 other mums with bfing but really got so exasperated with the shitty system that she suggested she give formula.
...maybe she got so tired and exhausted with the night feeding she decided to try formula as she heard it helped get the baby to sleep longer.
...maybe she was on medication which she was told was incompatible with bf, rightly or wrongly.
...maybe she was told she didn't have enough milk, or she thought that cos the baby kept crying she didn't have enough milk.
...or maybe she thought that as formula is so close to breastmilk it would be easier to just give formula.