Because the alternative to the nuclear plants that have shut down has reliably been ramping up fossil fuels. Note that nuclear has the longest of power up/down

Because the alternative to the nuclear plants that have shut down has reliably been ramping up fossil fuels. Note that nuclear has the longest of power up/down cycles, so it's mostly practical for providing the base load. I.e. you cannot practically replace nuclear with solar, as its home turf is night time.