How do you explain that most countries that want to become nuclear powers aren't able to do it?
How do you know this?
The thing is that most countries don’t want to become nuclear powers because there is a carefully setup and maintained set of incentives against it. This involves both carrots (you get good things if you don’t want nukes) and sticks (bad things happen to you if you do want nukes).
This set of incentives failed a few times and that is how we got nuclear proliferation. Almost everyone who wanted nukes enough despite this set of incentives were able to do it.
I can't know for sure of course. But it seems reasonable to me to think that at least most of the dictators would want to have nukes to keep their hold on power. And there are many.
https://worldpopulationreview.com/country-rankings/dictators...
Unless you have a clear and immediate use, having a nuclear program seems like a very expensive money sink.
And because all the other countries actively work against you getting the bomb.
The calculus is not simply 95% brain drain so 5% not drain so 5% will behave in this way. There is correlation between being in each group.
Or perhaps that supports your point. The smart people with fluid morality are vacuumed up by the Metas of the world, leaving none that would build a nuke for a third world dictator.