Can I say all of the above lol.
If i had to pick one I would say cultural influences/expectations & advice from family & friends. This is just from my own experience but the people I know who didn't breastfeed or only brestfed for a short time were mainly from families with a culture of not breastfeeding. A couple of examples - a friend who was using artificial milk by the time she left hospital with her first born had been told that none of the women in her family could breastfeed. With her second after encouragement from midwives and me she breastfed successfully until the third trimester of her third pregnancy.
SIL was told the same story by MIL - that she wouldn't be able to breastfeed because MIL couldn't. She started breastfeeding successfully but after a few months when her baby was going through a growth spurt and was waking up alot in the night MIL told SIL that she couldn't make enough milk and that she should give formula and do CC. Sadly SIL followed MIL's advice and that was the end of her BF relationship with her daughter.
In contrast I always believed that I could and would breastfeed because that's my family culture. I had my share of difficulties especially with Imran and his wonky tongue but we got there in the end. With Imran formula was nearly my undoing - MIL left a can in my house and at about 10 days with scabs on my nipples, a sore c-section scar and almost no sleep it was incredibly tempting. Luckily my DH talked me out of it.

FWIW I think that breastfeeding needs to be promoted realistically. In my ante-natal classes they showed us holds but they never told us that it could be difficult and excruciatingly painful. If women were more prepared for the difficulties of the early days they might be more inclined see it as normal and to continue instead of assuming that they couldn't do it.