Why did the Anti-Ballistic Missile Treaty limit missile defense systems?

The Anti-Ballistic Missile Treaty (ABM Treaty) limited missile defense systems to reinforce the doctrine of mutual assured destruction (MAD).12

By prohibiting the deployment of nationwide missile defense systems, the treaty ensured that both the United States and the Soviet Union remained vulnerable to the other’s strategic offensive forces. This vulnerability was considered essential to the strategy of deterrence; if neither side could effectively shield itself from a nuclear counterattack, the prospect of total annihilation would prevent either superpower from initiating a nuclear conflict.12

Essentially, the treaty prevented a new phase of the arms race that would have occurred if either side attempted to neutralize the other's retaliatory capabilities through defensive technology. By keeping both nations subject to the deterrent effect of the other's nuclear arsenal, the agreement sought to maintain a stable, albeit tense, strategic equilibrium.312