That part was never made clear, no. If the writers simply say "He cannot feel empathy or affection" that isn't a proper explanation, that is just a statement. The explanation would be why he can't feel that, backstory, details, reasons, etc. We are told/shown some things about Zenos, that he is this or that, or can do this or that, but we aren't given the reasons as to why. Some people are more than fine with just this much, they don't care to know more. Other people, like me, would like proper explanations and details.