Its a limitation, nobody wrote code to handle > 8bit for Cirrus.
Actually, we tried, years ago. One big problem was big- to little-endian byte swap, which rendered dog-slow on the Amiga.
I had some sort of 16- and 24-Bit "pnmtoCirrus"-display tool, which directly poked into the frame buffer. I lost the source over the years, though. Required kernel hacks as well, to implement some io-controls (display on and off and such).
BTW, X11 used to be 8Bit-only for decades.. -- Markus Illenseer