PWR are not actually good at that. In theory, with a liquid fueled reactor you should be able to be almost completely load following. Most GenIV reactor that ar

PWR are not actually good at that. In theory, with a liquid fueled reactor you should be able to be almost completely load following. Most GenIV reactor that are trying to go threw regulation is able to load follow at least as good or better then PWR. The reason we mainly use PWR is because the military wanted them for submarines and the government and industry had a massive program them into commercial use. From there we got a large amount of legacy, knowledge and infrastructure around them. After 3 Mile Island the nuclear regulatory system changed and regulation were made technology specific. So even if you wanted it would be practically impossible to get a license for anything else. Thankfully this has now been realized and they are changing their regulation to make them less technology specific.