system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some countries still struggle to make nuclear weapons?

We hear pretty often than Iran is attempting to enrich uranium and make nuclear weapons. The same has been said of some other countries and there are countless other nations that would like nuclear weapons but can’t make them.

How is this the case when this is 80 year old technology at this point? If we could make them in 1940, shouldn’t even technologically behind the times countries be able to make their own now? Was 1940 America really more scientifically capable than large African or middle eastern countries are today?
gpt
It’s not a matter of scientific difficulty, the process is well known and you can look it up online.

The difficulty is mostly in procuring the materials and equipment against the will of the international community.

You need enriched uranium and a large number of centrifuges to refine it to weapons-grade, but these aren’t easy to come by.

When your geopolitical enemies embargo these items and dispatch spies to disrupt the process, you’re forced to procure equipment through other unsavory channels and try to do this work in secret.

Iran has a lot of very powerful enemies and very few friends, and they can’t just go buy nuclear weapons equipment.