The whole problem is that the device runs iOS rather than macOS. This means that supporting such devices isn’t as simple as throwing a switch when compiling the code. The entire ZBrush code would actually have to be rewritten in order to provide the support.
This in turn puts iOS devices in the same boat as Linux or other operating systems. It’s not a matter of not wanting to do it but rather one of having to determine how much time is required and whether that same time would be better spent developing new features. Remember that not only is the initial port to the operating system a factor, but that it also requires additional development time for all future features as well because those most then be coded and optimized for three operating systems rather than two.
I’m not in any way implying that such support won’t happen. Pixologic simply does not comment on what might or might not be planned for the future. I’m simply pointing out the technical hurdles and the long term impact of such support.