Ugin understood that the visible Eldrazi titans were extensions of beings existing beyond ordinary planes. Destroying what appeared on one world might have consequences no one could calculate. With Sorin and Nahiri, he chose containment and used Zendikar’s leylines as the power behind a vast hedron network.
Nahiri stayed to maintain the prison on her home plane. When it weakened, her call for help went unanswered and she repaired it alone. Much later, Nissa broke the remaining failsafe because she believed freed titans would leave Zendikar. Instead, Ulamog and Kozilek remained to feed, while their broods converted living regions into distorted waste.
Responsibility for the disaster could not be assigned to one decision. Jace and Chandra had earlier helped open the Eye of Ugin, Nissa released the final restraint, and the ancient alliance had built a solution around promises that failed. Zendikar’s people paid for choices made by travellers who could always depart.