One of which is probably piracy, that is simple enough to overcome as all "new" macs (g3 blue and white onwards) support usb in the open firmware so it would be simple enough to sell os4 with a usb dongle which would replace the need for the a1's uboot rom.
The companies involved may not have yet accepted this, but I think the OS4 "market" has boiled down to the point where piracy isn't an issue. The only people left who even think about OS4 are those who are more than anxious to pay money for it, and those who might have been interested years ago but now wouldn't run OS4 if you paid them to.
