Many Americans make the assumption that a person cannot be moral unless he subscribes to a religion. But philosophy is replete with ethical systems other than divine-command theory, some of which have been around for thousands of years. Why haven't teachers of philosophy been able to teach or convince the public that being moral does not necessarily depend upon believing in a divine being?
I think there are probably two main reasons: (1) Philosophers don't generally speak to or write for the general public, and most are not suited to the role of public figure . Religious leaders (pastors, priests, ministers) have an opportunity every Sunday to speak to a much broader range of people. (2) Philosophers have little beyond argument to support their view, whereas religious leaders can encourage belief in their views by promises of good things (heaven, divine forgiveness) and threats (punishment, hell). These considerations seem more than sufficient to explain the phenomenon.