I think mis-information from health professionals is a huge barrier, alot of them really don't know what they are talking about.
For example, a friend's new bub wasn't putting on weight and she couldn't express much, so the health nurse told her to not feed at all overnight and in the morning there will be enough to express for a full feed??????? Terrible advice, night feeds are the most important in building supply and some women just cannot express no matter how much milk they have. Needless to say, she couldn't express anything in the morning, thought she had no milk and put her bub on the bottle.
When my ds was 5 months his weight gain slowed considerably, I was told to comp with formula which I only did once thank god, and then I called the ABA. They were great and told me that it is normal for bf bubs to slow with weight gain at around five months, unlike formula bubs who gain consistently.
The health nurses just don't know what they are talking about, and are putting off so many new mums who are very emotional anyway, and making them feel inadequate so they give up.