Infocom ported their Z-machine to nearly every viable platform in the 70s and 80s. Zork runs on around 18 different 8/16 bit platforms using earlier versions of the Z-machine. Why was Z6 the graphical version of the Z-machine ported to the Apple II, but not the IIgs?
I suspect it was too late in the game for Infocom... The Apple II port of
YZIP, Infocom’s Z6 interpreter, was released in 1989 (as far as I can determine), and by then Infocom was only a publishing house inside Activision, and was about to be shut down entirely.
I imagine that, given the limited development resources available, Infocom would have preferred porting the interpreter to entirely new platforms (new to
YZIP anyway, such as the Atari ST), before spending time creating a IIGS-specific port (since the IIGS could run the Apple II interpreter just fine, in IIe mode). There were only four games affected by this anyway (Arthur, Journey, Shōgun, and Zork Zero)...
The IIGS did get specific ports of Z-machine interpreters later on, for The Lost Treasures of Infocom, but they only support Z3 and Z5 games (so Zork Zero, which was part of the compilation on other platforms, isn’t included in the IIGS version). I can only guess why these didn’t support Z6, but I suspect that the effort involved was deemed too great for a single game (LTOI 1 included Zork Zero, and the floppy-based releases of LTOI 2, which were only released on the Mac and PC anyway, didn’t include a single Z6 game).