Genuine answer: power. Power consumption is a hard constraint. The benefits of A64 have to outweigh the cost in power consumption, and the benefits just aren'

Genuine answer: power. Power consumption is a hard constraint. The benefits of A64 have to outweigh the cost in power consumption, and the benefits just aren't there. Coin cells, small solar collectors and supercaps are among the extremely limited power sources for many Cortex M applications.