NuBus machines have a different physical address space layout. They have a different interrupt controller. They do not have Open Firmware (which we leverage to get information about the machine). These are but a few of the technical reasons why we aren't able to support NuBus-based machines.
It is unfortunate, but it is simply not technically practical to recompile our operating system and make it work on every Power Mac system, since an operating system has to have intimate knowledge of the motherboard it's running on. Given the resources of our small company, we've decided to concentrate on PCI-based systems.