Why does fusing iron atoms in a massive star core stop supporting the star?

It consumes energy instead of releasing net energy, cutting off the star's primary energy source.

Iron-56 occupies the peak on the chart of nuclear binding energies. This means that while fusion reactions involving elements lighter than iron release net energy—energy that counteracts gravity and keeps the star inflated—fusing iron atoms together does not yield energy. Instead, fusing iron requires an input of heat energy to proceed. Once the core transitions to primarily iron, the critical energy source that balances the crushing force of gravity is suddenly eliminated, leading the star to stall and begin rapid collapse. This contrasts sharply with earlier fusion stages, such as hydrogen into helium or silicon into iron, which were exothermic reactions supporting the stellar structure.