Because the main reason IMO is the attitudes in the society we live in, which isn't convinced breastfeeding is even a positive thing really, and certainly not something that should be talked about in the community even in a respectful way to all mothers.
I simply don't buy this. Our society is overwhelmingly convinced that breastfeeding is a positive thing. You'd have to have lived under a rock not to know that there is scientific evidence that breastfeeding is helpful. And its talked about ad nauseam in communities of new mothers and in the medical community. There is no more convincing which needs to take place. That ship has sailed.
But what isn't talked about enough is that its convenient for some women its actually bloody difficult for many women to breastfeed. For a variety of reasons. Sometimes technical (tongue-tie/latching problems etc). Sometimes cultural (no-one else did it in my family etc) and often logistics (breastfeeding is pretty onerous in terms of time and the limitations on women and many women simply aren't prepared to sacrifice that much of their time). There is some support for this but it's at best patchy.
A lot of women go into new motherhood with very good intentions and then discover that breastfeeding is much harder than they expected. Some of them stick it out and all power to them.
Many of them, not unreasonably, weigh it up and decide they'll take a marginal downgrade to the quality of their child's nutrition in order to to stop spending every minute of their time obsessing over how to breastfeed. In order to be able to leave the house and not to have to be cluster feeding throughout the night. In order to not have to go and find a place to express at work every couple of hours. In short, to get their lives back. If the impact on a child's health of a mother doing this was really significant then we as a society would owe it to those mothers and those children to support them more comprehensively.
But we don't. We bully them and nag them and send them "well-meaning" stories about a study that's been published somewhere which shows another marginal impact on the child's health. We make them feel inferior to developing world mothers when in fact the entire infrastructure in these countries is completely different and you're not comparing like with like. Etc etc.
Going on and on and on about breastfeeding being a positive thing is really not the solution to this.