Iran is like the second/third most sanctioned country and they do not have their economy organized in a rational (socialist) way. The costs for their projects are higher than countries that can freely trade materials and technology, they face extensive sabotage and large conventional military threats. Unlike the DPRK (until fairly recently, not while they were doing their nuclear program), Iran is consuming their weapons in proxy and direct confrontations and has to maintain and expand that production at the same time as any nuclear weapons programme. They also want better air defences & air power, better naval assets, and longer range missiles so the retaliatory threat of a future nuke can reach European-Yankee missiles instead of just Israel.