I'm still assuming that there is an implicit or explicit but not known agreement between a number of developed countries not to pursue nuclear energy programs further.

ex: If we all solve all our energy problems with nuclear it makes it look like "Why don't we give it to the little guys," / makes it indefensible not to / still talk about climate change. But we don't want to because giving every country in the world (or even a lot of them) the ability to make nuclear weapons means that you have even more chances for "something to go wrong," which can mean the end of the world.

So basically that's why most countries won't do it even though they could if they wanted to. The possibility of the world more or less ending outweighs the less concrete on the horizon maybe we can deal with another way threats of climate change.