I think it has to do in part that almost 80% of the black population is overweight and in a lot of the culture "big asses, thighs and breasts" are seen as a good thing as opposed to other cultures where being thin is a good thing and a sign of wealth. Since they've glorified being fat for so long, its now seen as a completely regular thing and its actively sought after. In addition is a lot of the ignorance and lack of education. A lot of people live life without understanding the basics of nutrition and the importance of health.