The conventional explanation of more no of neutrons in nuclei is that because protons repel each other through the electrostatic force they must be diluted by neutrons.
The conventional theory of nuclear structure maintains that all nucleons are attracted to each other through the so-called strong force. And they will balance the electrostatic repulsion of proton - proton.
As you get to heavier elements, with each new proton you add, there is a larger repulsive force. The nuclear force is attractive and stronger than the electrostatic force, but it has a finite range. So you need to add extra neutrons, which do not repel each other, to add extra attractive force.
I have kept the answer at the basic level. And not went to more rigour and quantum world.