Why does nuclear fusion stop when the massive star core becomes pure iron?

Fusing iron consumes energy instead of releasing it.

Iron-56 represents the most stable configuration among all atomic nuclei. When fusion attempts to proceed beyond iron, the reaction becomes endothermic, meaning it requires an input of energy rather than releasing the energy that stellar cores depend upon for outward pressure. Before the iron core forms, successive fusion stages, from hydrogen to silicon, release energy, which successfully counteracts the immense gravitational crush. Once silicon burns into iron, the core loses its internal energy source. This termination point is terminal because the outward pressure necessary to maintain structural integrity vanishes, allowing gravity to proceed unopposed.