Lack of support must be an issue in why people start but then stop quite early. Plenty of encouragement to try BFing, but if things are difficult, it is so hard to get help. I was committed to breastfeeding, it never occurred to me not to as am from a BFing family and hadn't really considered formula, yet I came close to stopping because it was so hard and I became quite upset about it. I couldn't have done it without lots of help from DP, who had been able to take extra time off work, and hours spent reading and posting on MN threads and Kelly Mom. It did all click eventually, at around the 6-8 week mark, but if I had been less determined or had less time, I would have moved on to formula.
Cultural expectations. Lots of people do what their families and peers do. And in the wider community, although people are generally fine with newborns being fed, some shift seems to happen around 4 to 5 months.
Some of the information you are given around feeding isn't helpful. Makes it seem easier than it is. I would like alternative leaflets to be printed, "it might hurt, it can be really hard, you may want to give up a million times, but if you do get through that, you have free food for as long as you want it, and no need to sterilise stuff, so on balance it might work out for you".