One radical answer that question which is often neglected for the facile "regulations" explanation is that we quit building coal burning power plants at the same time we quit building nuclear plants because the steam turbine and heat exchanger cost too much compared to natural gas plants.

The timing undermines this theory. The US added one nuclear reactor to the grid in 1996 then zero until 2016 (sort by first grid connection date here):

https://pris.iaea.org/PRIS/CountryStatistics/CountryDetails....

The US built an additional 58 coal generating units between 1995 and 2009 (see section "Age comparison of coal plants"):

https://www.gem.wiki/Existing_U.S._Coal_Plants

Combined cycle natural gas units were already cheaper to build in 1995, but the gradually rising natural gas prices over the next ~12 years meant that coal could still compete on cost for electricity generation. The cheap fuel for coal units counterbalanced the slower, more expensive construction process. It wasn't until fracked natural gas drove fuel prices down that coal unit construction ended in the US.