Nukes have the extra ball-and-chain of huge capital cost to amortize over all the kWh they will produce over their lifetime. This means that if you end up operating only half time, there are half as many kWh to amortize over, and each kWh costs even more, making you even less competitive. (Operating a steam turbine less might reduce maintenance cost, but probably not if you are starting and stopping it every night, or if you keep it cycling to avoid that.)

So the expectation is that nukes will very quickly become unable to produce enough revenue to pay for continued operation. Then they are mothballed. Suddenly, all that capex is amortized over a much smaller lifetime kWh total, and the actual cost of every kWh ever produced jumps instantly to greater than you ever were able to charge for it. Then you go into receivership, or anyway write off that capex you thought you had invested, but really just poured down a hole.