To make bombs. And what's changed now is we have enough bombs.

Seriously, even the reactors that aren't expressly breeding fuel were designed with an eye to the technology and as part of programs in the 50's-70's to build out expertise in this critical technology area of national security.

Countries wanted bombs. To make bombs you need a nuclear industry. Ergo, everyone who wanted bombs built civilian reactors, without exception.

renewables were 10-100x times more expensive back then

I think a lot of countries built reactors when the economics of Nuclear reactors as well as the economics of renewables were unknown with respect what they would look like in scaled deployment. Modern looks at the operating costs vs time to build, vs modern construction practices from the US, France, China, et al seems say the economics of Nuclear reactors have stalled or increased, while the economics of renewables continue to make accelerating improvements.

Yes we have more stringent safety requirements on nuclear, driving up costs

Producing plutonium, I suppose.

Yes. Regulations.

The economics are what they are do to the legal burden and regulatory overhead more than the technology itself, at least here in the US.