6 bits doesn't pack well into an 8/16 bit arch though, you can store two pixels per 8 bit, which means smaller LUTs etc, whereas 6 bits doesn't match as well. How would you represent the 6 bits per pixel in the code/data ?
mode x is byte per pixel, has page flipping and it fits into the cache arch well, so thats what makes it better than mode 13. Perhaps you're mixing up modes? , MCGA mode is the normal 256 colour 320x200 that mode x replaces, not the 16 colour old VGA mode. you can write 6 pixels at a time, and it optimises really well around rep stosb's etc.
since i think we're mixing up the mode 12 vs mode 13 and mode x, there are some differences, so we might be talking at cross porpoises.