The teachings of Confucianism appeal to many of the people of that age (and now) because the teachings applied to life at a level that even those of a lower caste could understand. From my perspective, if I had been exposed to the teachings of Confucius at that time I too would have considered "converting." All of his lessons and speeches dealt with the type of experiences one could normally expect to encounter during a lifetime, and from there clear and concise rules are laid out to become the basis for actions.
Confucius decided not to focus on matters of metaphysical importance in his discussions, though he was well-known for his ability to converse as deeply as Socrates, because he felt that things of that nature would not be applicable to his teachings. That leaves followers of Confucius quite a large metaphysical gap to be filled. I believe that this method of teaching leaves far too many gaps when it comes to matters of deeper questions about life and focuses too heavily of the physical world to be classified under the banner of religion.
I must commend the followers of Confucianism for the dedication it takes to follow all the constraints set forth in Confucius" Five Constant Relationships in order to condition the mind to the type of response that would elicit maximum fulfillment in life. I hope that any comments that I have make to the effect that Confucianism isn't a true religion will not be received by too negative of a response. I have nothing but respect towards those that feel strongly about a certain belief, or set of beliefs. In the case of Confucianism though, it is possible for someone to still believe in God and follow Confucius" rules for living, or Judaism or Buddhism for example. The teachings of Confucius are well thought out, and they express his desire to focus on the physical realm by his shift from worship of ancestors to worship of elders and other such changes in focus.