Key point: Saudi Arabia wanted long-range missiles and it got them. It turns out that when you really want something, you don't care who helps you.
You would be hard pressed to find two more determined foes of Iran other than Saudi Arabia and Israel. The latter country has long been perturbed by bellicose anti-Israeli rhetoric from Tehran, and has unleashed hundreds of air strikes and artillery bombardments targeting Iran’s efforts to arm Hezbollah forces in Lebanon and Syria.
Meanwhile, Riyadh appear to see itself as engaged in nothing short of an epic struggle for dominance of the Middle East, and has oriented its foreign policy around combating the perceived Iranian menace, even in places its influence is moderate at best.
Iran hawks are preoccupied by the possibility of an Iranian nuclear weapon—a weapon which, given the limitations of Tehran’s air and sea forces, would need to be delivered by a ballistic missile. Iran’s continuing development of such missiles has been proposed as a casus belli, and was cited to justify the U.S. withdrawal from a nuclear deal struck in 2014 (the deal constrained Iran from developing nuclear warheads, but not ballistic missiles to carry them in). It’s often ignored that Israel and Saudi Arabia themselves maintain some of the largest ballistic missile arsenals in the region—the latter of which is the subject of this article.