Nucleosynthesis


Deuterium is fragile
At temperatures above a a billion degrees Kelvin it is knocked apart faster than it can form.

As the temperature drops below that threshold the lighter elements form rapidly - almost all the available neutrons are "mopped up".

Nucleosynthesis would proceed all the way up to iron if the density and temperature remained high enough...
But the continued expansion "freezes out" the composition in a funny (dis-equilibrium) state.