Regarding the general population, I'm afraid I can only speak in very general terms, as that really is not my area of expertise. I recall a religion professor in my undergraduate days repeatedly stressing the adaptability of Manichean religion as an important part of it's success. The idea being that Manichaeism could, as it were, take on some of the character of the religions with which it competed. But I will have to leave an evaluation/elaboration of that claim to other more knowledgeable than myself.

I can, however, speak a bit more about the intellectual appeal of Manichaeism. Philosophically, Manichaeism is ontologically dualist; to the Manichean, there were two fundamental substances or principles, one good and one evil. This allows the adherent access to some pretty natural answers to prominent questions.

Why does evil exist? Because there is a positive, active evil substance at work in the world. The evil substance and the good substance are at war with one another, but neither has destroyed the other.

Why do human beings sin? Because we are part of the war between the positive evil and good substances in the world. We are conflicted because our nature is conflicted. We are, as beings, the light of goodness ensnared within material bodies, here thought of as being a product of the evil principle. Just as the good and evil substances are at war with one another, we are at war with ourselves.

Now, why does this sort of thing matter? Because these are questions that an intellectual religious person would want to solve. An intellectual searching for the true religion would want to be able to make sense of the existence of suffering, of the reality of sinfulness, and in general of human and worldly imperfection. No one, perhaps, was more attuned to these problems than Saint Augustine (354-430).

Augustine felt a few things very, very deeply. First, he worried endlessly about the sinfulness he felt in his own being. A particularly worrying example is a theft of some fruit he and some friends committed as young men. Now, this seems a trifling incident to most, as it is basically the equivalent of a bunch of teenagers or college kids committing a dumb, but not particularly heinous, crime. But what worries Augustine is that there was, apparently, no perceived good at play: "I wanted the stealing not the thing stolen. I wanted the sin." (Confessions 2:9). Other sins are explicable because they contain something good, which we mistakenly value (or attend to) rather than considering the situation in full (Confessions 2:13). But to chose to sin for the sake of sinning makes no sense.

Manichaeism, of which Augustine was a devotee for a long period early in life, has a ready answer. You chose to sin for the sake of sinning because there is an active, evil principle in the world and within you. Christianity, by contrast, seems to be in trouble. After all, if there is one all powerful, all good being then how on earth could everything be so ruined? How can we make sense of our fallen nature, of sin for its own sake, of pain and suffering? Satan is of no help in this regard, because, in the first place, Satan is not coequal with God, and in the second place Satan's own fall requires explanation.

So, if you were the sort of person who took seriously the problem of evil (as it is now called), then Manichaeism was an attractive doctrine. It readily seems to explain a problem with which Christianity had considerable difficulty. Of course, then as now, such difficulties did not matter much to the majority of Christians who weren't of a philosophical bent. But for someone like Augustine, solving the problem of evil led him first to Manichaeism and then, when he grew dissatisfied with that religion for other reasons, he nevertheless found it necessary to resolve the problem of evil before he could assent to Christianity. He eventually did, at least to his own satisfaction, via Christian neo-platonism.

*Edit: Sources

Primary:

• Augustine's Confessions

Secondary:

• John Rist's Augustine: Ancient Thought Baptized. See esp chapters 5-7.

• G.R. Evans' Augustine on Evil

• For a less daunting read, the Stanford Encyclopedia of Philosophy entry on Augustine deals extensively with both his relationship to Manichaeism and his eventual Christian solution to the problem of evil. See, in particular, sections 1 and 4.