Nuclear is on the decline for the same reason we aren't fighting climate change in other ways; it's expensive. Building new nuclear powerplants is far more expensive up front than other methods of generating power so a new power plant won't actually start seeing a return on investment for a decade at least. Shareholders now a days are allergic to anything that doesn't see a return on investment within 3 months. Nuclear power plants also take much longer to build than other power plants. Don't take this as me being antinuclear. I would have a nuclear power plant in my basement if I could; but nuclear power does have it's disadvantages.

As far as geoengineering goes it's just that our governments are already dramatically mishandling the current situation. Do we really want to risk them making it much worse in new and exciting ways when we could just be lowering our CO2 output instead?

"Nuclear costs too much, and nations can't be trusted to not cut corners" is basically what it comes down to for Nuclear. As much as I will always always always be a support of Nuclear, humans probably won't handle it properly. We haven't in the past, and I wouldn't trust that we've changed enough to handle it properly in the future.