It has often been noted that the bulk of any rational discourse tends to be haggling over definitions. Thus, before I can explain why atheists and agnostics cannot be moral people, I should first examine what it means to be moral. I am reminded of a quote, which I paraphrase, the ethical person knows what should be done, the moral person does it. The simple answer to the complicated question is that the moral person is one who consistently tries to do the "Right Thing."
Therein lies the problem for the atheist, there is no right thing. There are nice atheists, compassionate atheists, and generous atheists, but no moral atheists. Indeed, how can one even build a morality from an atheists viewpoint? One might do something to benefit from it, which is self interest, or because it seems natural to do as it confers a reproductive benefit to your social group, which is evolutionary biology. However, I believe we can agree that we would not want self interest or a biological imperative to be what we mean by moral.
Even attempting to define what a good person is in the language of atheism is difficult. We might list qualities that we enjoy about them, we could do the same about a treasured car or other possession. Does then saying that someone is a good person carry no more weight than that a vehicle is a good car? Should goodness be a measure of how pleasing and or functional someone is? If so, I would argue again that it is divorced from any common sense notion of morality that we posses, for one might call a particularly canny criminal a good felon, meaning that they are quite functional at what they do, and by no means wish to attach moral approbation to the comment.
It seems the closest an atheist will come to morality is if they believe that there are some actions which are, in some sense, inherently right or wrong, such as treating others equitably or lying. However, should such a person be out there, I ask you, if there is no higher power or being to dictate such things, what makes an action inherently right or wrong?
Certainly those of us who do believe in a higher power have our share of moral quandaries. As Socrates taught, saying that the higher power dictates what is good also robs it of much of its customary meaning, for then murderous rampages would be good if one's god decreed it so. That said, I can see no reason to even think about right and wrong without a belief in some structure of ethical obligation upon the universe, and that is why, although I appreciate the friendship of many of them, atheists are amoral beings.