Why was the crystal clock frequency for the original Macintosh (128k,Plus, etc.) 15.667 MHz, instead of a multiple of 8 MHz (which was the maximum clock frequency for the speed grade of MC68000 choosen), or a multiple of NTSC color burst frequency, as was the predecessor Apple II/III line of computers.
15.667 Mhz is about 2% less than 16 Mhz. So for purposes of "multiple of max. MC68000 frequency", it is close enough. The Mac wasn't competing based on being the fastest machine, so 2% was just not a big deal. And most of the competition was either older 8-bit or Intel family, not MC68000. In fact, the 8088 in the original IBM PC was clocked at 4.77 Mhz, which is about 5% less than the 8088's rated speed of 5 Mhz, so Apple was (sort of) copying IBM in this one aspect.
The main driver seems to be the video. See answers to this other recent question What was the screen refresh rate of the Lisa and original Macintosh? for more details. It gets complicated because Apple was doing some innovative things with video & graphics. Where most other computers of that time were designed (or could be relatively easily adapted) to run on ordinary monitors, Apple based the Mac on using a built-in, Apple-supplied, monitor and nothing else.