They can, it's just that fusing iron or anything heavier takes more energy than it releases, because of the size and stability of the nucleus.

So usually iron and heavier elements mean the star is in its final stages. There's a LOT of energy in the core of a star, so heavier elements can still fuse, but they're absorbing more than they're producing. Once the star runs low on fuel that produces more energy than it takes to fuse, the total temperature starts to decline, eventually leading to collapse or implosion.

The heaviest stuff is only produced in the most intense parts of supernovas.