The programs are very different. There was a significant amount of work that went into (and is still going into) developing UVO. Where Optivox was a voxel modeler that had optimization methods, it was far too much work for me to do as one person, and I couldn't deliver that. UVO has completely different features and focuses much more on optimization of voxel models.
Secondly, everyone that had purchased Optivox directly received a key for a free version of UVO. For the people that received the key through bundles (of which I get no profit), it seems entitled to expect a new program when it was released months after they first received Optivox.