It is interesting that "Ultra-Supercritical coal (USC)" is the cheapest coal technology listed by the EIA. It is not cheap at all! It is expensive compared to old fashioned coal fired generators. I do not know what the story is. I looked up this technology in places like this:
You can see it is very efficient. Perhaps it is more cost efficient over the life of the plant? Maybe less polluting?
Maybe the government wants all new plants to meet the highest standards for efficiency and pollution. As I said, I don't know, but the EIA data all comes from industry sources, so I guess this is the only kind they are still building.
Anyway, this is the pattern you often see in the last stages of an obsolescent technology. The last generation of sailing ships were gigantic fore-and-aft schooners. The last ones were made around 1920, for bulk cargo. Why schooners? Because they needed many fewer sailors and they were a lot safer to operate than square rigged ships. You could raise or lower the sails from the deck without climbing the mast. You could do it with a deck engine. The sails were so big, you probably could not do it any other way. In other words, the last sailing ships had many of the advantages of steamships. They borrowed technology from their modern rivals, in the final flowering of the technology. Similar to the way hybrid automobiles use electric car technology to extend the life of the internal combustion engine a few more years. It has not worked out as well as Toyota hoped. That's why they are licensing the technology to other auto companies for free. Hoping they will buy parts. It does not seem to be working. I don't see new hybrids coming onto the market.
The last generation of minicomputers in the 1980s tried to incorporate some of the advantages of microcomputers and personal computers.