Why does nuclear fusion cease in the core of a massive star that develops an iron core?

Fusing iron consumes energy instead of releasing it

Massive stars progress through fusing progressively heavier elements in their cores—hydrogen to helium, then carbon, neon, oxygen, and silicon. This sequence generates the outward pressure necessary to support the star against gravity. However, the process stops abruptly and fatally once the core converts entirely into iron. Fusion reactions involving iron are endothermic; rather than releasing the massive amounts of energy required to sustain outward pressure, the process of fusing iron actually absorbs energy from the core. When this energy sink develops, the outward fusion pressure vanishes instantaneously, gravity gains absolute dominance, and the core undergoes a catastrophic collapse in milliseconds.