Ostensibly the reason is that the ROM chips have to run in lockstep with the CPU clock, so the CPU may need to be downclocked if cheaper ROM chips can't keep up

Ostensibly the reason is that the ROM chips have to run in lockstep with the CPU clock, so the CPU may need to be downclocked if cheaper ROM chips can't keep up with it, but I don't know what the actual cost difference would have been at the time, if any. Maybe all of the chips were capable of 3.58mhz and Nintendo was just gouging.