It really seems to be common sense. Obtaining a nuclear weapon will threaten their power and hold on the country by bringing more hardship and turmoil to their country.
Funny this is one issue where I think Donal Trump could be a fun president. He wants to sit down with them and have a business discussion, not just levy them with sanctions. I really think if we negotiated, the powers that be would see there's a lot more to be had by not building weapons.