it has to do with the fact that the nucleus of an atom is consist of
neutrons and protons. But the protons are all positively charge and thus
will repel each other due to electrostatic forces. However, once the
number of nucleons reaches "saturation" (there is enough of them), then
the strong nuclear force takes over and overcomes the electrostatic
repulsion force.
Thanks & Regards
Mukesh Sharma
askIITians Faculty