A balance of the forces which expand and condense form is required for those forms to remain intact. Atoms of lighter elements contain nuclei where there is a nice balance of forces, and this allows them to be especially stable. The Fusion of light nuclei releases energy, and creates heavier elements in the process. As one goes up the Periodic Table, however, the amount of energy required to evolve a nucleus for a given atom increases, and the amount of excess energy liberated during Nuclear Fusion decreases instead. Past the element Iron, one needs to put more energy in than one will liberate with the reaction.
Heavier elements are increasingly unstable.
Many will break down spontaneously, and these elements are said to be radioactive. The individual spins of the particles which comprise the nucleus of an atom of Uranium, for example, tend to result in forces which will pull that nucleus apart. But this makes Uranium very difficult to form, and in the presence of high energies and constant bombardment, it is not likely to form at all. In the early universe, the conditions were not favorable for heavy elements to exist. The elemental building process did proceed, however, for about a million years before decoupling.