I voted for Health Professional influence, but I think its really a combination of things, probably these 4 would be the main ones in my experience.

Lack of education
Health professional influence e.g. MCHN, Paed

Lack of education, both on the part of the mum and contributing to the health professionals "advice".

I know of one OB who frequently tells mums to stop breastfeeding at their 6 month check if he decides their weight gain is not up to scratch, when often its fine for a b/f baby.

A lot of mums have no idea. Most in my mothers group did not know that the charts (until recently) in the baby books were based on white, middle class American FF babies.

I have also met a lot who are scared of the ABA. I mean it, the term "breastfeeding nazi" was used, and they thought that they'd be frowned on for having problems.

I was very, very lucky that all of the midwives when my son was born were supportive of me breasfeeding, and that he was a big baby.
My milk did not come in until day 5, and until then we were hand expressing and catching it in syringes to feed DS. He didn't attach properly, breastfeeding DS was extrememly painful for me and frustratig for him.

The oncall paed wrote him up for comp feeds.
The midwife and the LC discussed it with me, and we decided that he didn't need them just yet, he was getting enough, albeit barely.

I stayed in an extra day at the Breastfeeding day stay (not enough of these available IMO. I have been 2 two different hospitals for them, it was great) and had another 5 or so visits after we went home.

I was lucky to have a supportive MCHN, who could recommend a pro-b/f paed. for me to see.

Unfortunately not everyone else is as lucky as I was.
I think all mums and babies deserve to have this care and these opportunities.
It shouldn't come down to the luck of the draw.