What do they have to gain by having nukes that's worth losing all the business they've just started to regain, and much more? Damaging their relationships with not only the US and EU, but also Russia, China and other less choosy countries. Losing the moral high ground that, at least domestically, Khamenei can still claim by publicly denouncing nukes as wrong and illegal under Islamic law.

Don't get me wrong, I'm sure they'd be happy to acquire nukes if they could do so at a small enough price, and justify it by claiming the purpose to be deterring rather than attacking - which incidentally I do believe would be mostly true. And hence it's important for them to remain close to that threshold from where they could quickly get to that point if the opportunity presented itself. The job of the international community is to keep the price of actually crossing the line high enough - and that's working rather well.