Traceability - supposedly it's possible to accurately determine the source based on the unique fingerprint in the material. Not sure if this applies to post-detonation in the case of a nuclear weapon. In theory, since very little of the material is actually fissioned?

You wouldn't want the ire of the nuclear powers in the world, if you are Iran.

https://www.sciencedirect.com/science/article/abs/pii/S01491...

It takes a lot to produce weapons grade plutonium, you need to run a reactor for some time and do quite a bit of complicated and dangerous post processing. While it's not something with a traditional price tag, estimates are that weapons grade plutonium costs about $4000 per gram to produce in the US. Further, if anyone notices you starting up the reactor to do so, that could be a massive diplomatic problem - something you especially don't want when the middle east is currently in a highly tumultuous state.

Presumably the Yakuza got access to the plutonium by illicit means that were substantially cheaper than producing it themselves, meaning they can sell it for what would otherwise be a loss and still make a nice profit. It's quite reasonable that if Iran wanted to say conduct some nuclear weapons research while appearing to comply with IAEA requirements that buying some plutonium off the black market would be the most economical option.