I’ll take a stab at simplifying the reasoning

Everything up to that point (hydrogen, helium etc) the gravity / force of a regular star is strong enough to overcome the natural repelling force of the 2 atoms. For iron it isn’t.

Or you could think of burning things - Think of ash in a fire. A regular star has a “temperature” that can burn elements up to iron, when it gets to iron it starts to cool because iron is like ash (eg it’s not hot enough to burn it so temperature drops due to fuel starvation). That’s why we have other elements - certain situations can make special stars that are hot enough to burn that ash into other elements. Easy comparison from home fire / stove burning paper to wood to coal or to a furnace melting metals.