The World Health Organization says on breastfeeding that:
Breastfeeding is the ideal way of providing young infants with the nutrients they need for healthy growth and development. Virtually all mothers can breastfeed, provided they have accurate information, and the support of their family and the health care system. Colostrum, the yellowish, sticky breast milk produced at the end of pregnancy, is recommended by WHO as the perfect food for the newborn, and feeding should be initiated within the first hour after birth. Exclusive breastfeeding is recommended up to 6 months of age.According to a 2005 survey, only 8% of babies (less than 1 in 12) born in the UK are breastfed exclusively for the first six months, despite the fact that 84% of mothers "said they were aware of the health benefits of breastfeeding."
This seems to be typical of developed countries.
Why? Read the article. I suspect that the ideas presented there are transferable to other public science issues.