Since he's part of a spin-off I'm not surprised. Most people haven't even heard that name or know who he is. If Genesis would be considered, Zack would have to be there already. I still envision a Dissidia Uni where all the major playable characters and major humanoid bosses exist. It might be pretty boring because I think the mannikins weren't supposed to be introduced until a later cycle.
Anyway, Genesis would open up a lot of characters from sequels, but since not many of the numbered FFs got sequels or spin-offs it wouldn't be balanced from one saga to the next.