Mostly it is the half lives of the reaction products, sometimes the half lives of the decay products or intermediate products. Ramping up and down was historic

Mostly it is the half lives of the reaction products, sometimes the half lives of the decay products or intermediate products. Ramping up and down was historically somewhat difficult, famously one of the contributing factors to the Chernobyl accident was the reactor having been turned down to a very low level on the previous day. You have to be careful not to accidentally poison your reactor during the day when the solar farms are chugging out megawatts only to not be able to ramp up in the evening when the sun sets.