I think that sometimes saying you 'couldn't' breastfeed clouds the issue a bit. As has been said repeatedly, the number of women who actually physically 'cannot' breastfeed is very low. Most of the cases involve women/babies who are having trouble with supply/latch/pain/weight loss etc. In these cases breastfeed probably could be successfully established if the mother worked really hard at it, seemed support and really put her mind to doing it.
But in reality when you have just pushed a baby of your vagina/had it wrenched from your abdomen, you are absolutely knackered and still in pain, you are sick with worry because they are not gaining weight, your nipples are shredded and every feed brings more pain than labour, you are adjusting to life with a new human to look after, or you are worrying about your other child/ren and that they are not getting enough of your attention because you are constantly breastfeeding AND there is an alternative to breastmilk right there that your baby will most likely thrive on......
Well then 'working really hard at and really putting your mind to' breastfeeding doesn't always seem like the best option. It's incredibly difficult to see a few weeks into the future where breastfeeding will suddenly click and become really easy, when you are in the midst of all the above feelings.
And I know from experience that all you want is for someone to say 'its ok to crack open the Aptamil' rather than saying 'if you stick at it a little bit longer it will get better, why don't you ring La Leche League or go to his breastfeeding group o Thursday'.
Switching to formula is nothing to be ashamed of. I was very able to breastfeed, I had tonnes of milk, and both my kids gained really well in the first few days/weeks. But there were many other issues going on, particularly with my first baby and I went onto formula quite quickly.
I could tell people that I 'couldn't breastfeed, but I'm not ashamed of my choice and I just say 'yeah, it wasn't for me'. In fact I am incredibly proud of my choice to ff because it has produced two of the worlds most amazing kids 