Just wondering why multiple-ISA cores in the same chip isn't more common. It would seem like a very useful feature. From either core(s), use the same periphera

Just wondering why multiple-ISA cores in the same chip isn't more common. It would seem like a very useful feature. From either core(s), use the same peripherals included in the SoC. And use the same peripherals / connectors / storage etc included on a board with that SoC. Just from a development perspective that could be very handy. How much silicon area to devote to each ISA's core(s)? Decide that depending on intended use cases. Edit: and with "multiple ISAs" I mean for application software. Not the 'hidden' housekeeping cores (different ISA from the main one) that are found in many complex chips these days.