What happened is nuclear reactor failures at Chernobyl, Three Mile Island, and Fukushima caused a huge public backlash, resulting in an actual mass anti-nuclear movement. Like I mean protests, political parties, the whole deal.

There was a huge popular push to decomission existing nuclear reactors, and in Germany the relevant political party became hugely successful and basically closed all their nuclear plants.

This is a big part of why the green energy movement, while enthusiastically endorsing solar/wind/hydro/geothermal/etc, doesn't really support nuclear.

Aside from all that stuff, the economics of nuclear fission reactors are just much more long-term than those other kinds of energy generation. Nuclear reactors take a lot of time and resources to build. Both in and of themselves, and to make sure everything is properly up to safety standards. That initial investment will of course be recouped as the power plant keeps running, but it takes years and years. Of course, this is mainly a "downside" because of our definitely very rational economic system, which is obsessed with quarterly profits and is apparently allergic to these kinds of longterm investments.

There is work being done on developing smaller scale fission reactors with fewer up-front costs, but public sentiment still seems to be against it. Research into nuclear fusion seems to be going pretty great (the stuff Helion's been working on looks promising), so if that comes through maybe we won't have to fight a tide of stupid public sentiment to get proper, stable renewable energy.