Because of the American Puritannical values, which dictate what the credit companies and advertisers are willing to do business with and the cultural zeitgeist along with it.
The Puritans were some of the earliest British colonists in the US, and were either thrown out of England for attempting a coup to replace the king with a puppet to force their more extremist form of Christianity on the country, or left by their own choice because they felt that the Church of England was too liberal. They were basically a bunch of prudes who believed that the human body and sex were shameful and disgusting.
This has led to the dichotomy where advertisers want nothing to do with sex/nudity, except when it comes to implied sex in advertisements. Because sex is bad, but it also sells, which is good.