The international community has been acutely concerned for many years about Iran's increasing capacity to produce material for nuclear weapons. With sufficient fissile material and a warhead design, Iran could use its existing ballistic missiles to pose a credible nuclear threat throughout the region. Consequently, after repeatedly directing Iran to suspend uranium enrichment, the UN Security Council decided in 2010 that Iran also had to halt all activities related to ballistic missiles capable of delivering nuclear weapons.
Now that serious negotiations are under way to curtail Iran's ability to dash for a bomb, seeking ballistic missile limits as part of a comprehensive nuclear deal would be unwise. Getting adequate and verifiable constraints on Iran's nuclear program remains the highest priority. To also demand severe limits on conventional weapons that Iran regards as vital to its self-defense would jeopardize the negotiations' key objective.
See full text at the Arms Control Association: Iranian Missiles and the Comprehensive Nuclear Deal