Who are the actors trying to advance the development of nuclear energy?
Primarily, the U.S. DOE through the Office of Nuclear Energy, with funding of about ~$1.5 billion per year. ~$1.5 billion is about equal to the combined spending on all spending on renewable energy technology innovation (wind, solar, water, geothermal, ~$700 million), advanced manufacturing ($400 million) and building efficiency innovation ($300 million, buildings are ~75% of grid demand). (This is EERE budget minus transportation and the weatherization program). Note the ~$1.5 nuclear estimate does not include the several billions in basic nuclear physics research including fusion funded through the Office of Science; the Office of Nuclear Energy is focused primarily on near-term demonstration of advanced reactor technologies.
To a smaller degree, other countries including Canada, the U.K, Russia, and China are also funding advanced reactor demonstrations.
Why don't most people trying to advance the development of nuclear energy care about effectiveness?
For the most part, those involved in the industry do. I think it is easy to misinterpret the voice of the industry with the din of nuclear bros turned keyboard warriors complaining how their one true hero was unfairly brought low by overly-risk averse regulation and unscrupulous environmentalists.
The reality is that advanced nuclear receives broad public support, and has for some time now . Safety regulations are becoming streamlined. Regardless, the nuclear-island and safety are a small component of overall plant costs, typically <20%, with most of the cost in the non-nuclear components of the cooling system necessary for any large rankine-style generator. See the EIRP report and the CATF report on advanced nuclear costs. The most optimistic projections are that new advanced reactors will costs $40-60/MWh in a decade or more. Compare that to current bulk power purchases at $15-30/MWh for wind and solar+batteries which are on 20-40% learning curves.
New reactors will likely never be cheap enough to compete directly with the combination of renewables, short-duration storage, and transmission on cost except in regions with particular climatic or geographic constraints. So that means they must compete in the firm generation market (supplemental power at peak times when renewables aren't producing) against a numerous array of other technologies, including efficiency, many with much lower projected cost floors. TerraPraxis is trying to get around the cost barrier by repurposing old coal plants that already have the non-nuclear island components built. They are relying on price drops of SMRs to make this feasible and are probably at least 5-15 years out from an initial demonstration project.
And why are they so rare?
Most of the people who are primarily concerned with decarbonizing the grid have looked at the economics of nuclear power and decided to pursue other grid technologies that they think are more promising. Those that have stayed with nuclear are particularly attached to it because they've spent their careers in the industry and don't want to see it die, think it is an important option to pursue for the sake of having options, or have some dogmatic reason why they prefer it as a technology.
Thinking more broadly, of all sectors that need to decarbonize, the electric power sector is the one that has been the fastest to decarbonize and has the most optimistic outlook. At the margin the most important innovation is happening in agriculture, industry, transportation, and carbon capture. Strategic thinkers see that and move to work on problems in those sectors.