The idea is simple. Because radiation is a much better energy carrier than mass. That is. It can hold much more hotter energy with it. Where as mass has different limits.
The way I see it. If you use mass like water. You are in fact using a poor functioning go between battery. This is similar to charging a power bank and then to charge the phone or computer with it. The efficiency this way is very bad.
I wonder if you just could use thermo photo voltaic to convert extracted energy to electricity directly instead. Close to the source or perhaps you could use a molten silicon based thermo energy storage device to capture heat at higher temperatures and then convert it to electricity using thermo photo voltaic.
So you have a smaller coal based power plant with molten silicon based thermo energy storage and thermo photo voltaic cells to convert the heat to electricity.