Because the coulomb force has a wider field of influence and heavier atoms have more neutrons than protons.
Strong nuclear force :
The strong nuclear force is represented by the force that holds protons and neutrons in atoms' nuclei. the things that are heavier than an atom of hydrogen. the center of the fusion of hydrogen and helium in the sun. The strong interaction, commonly referred to as the strong nuclear force, constrains quarks into proton, neutron, and other hadron particles. Neutrons and protons are held together to form atomic nuclei by the nuclear force, often known as the strong interaction. Heavier atoms have more neutrons than protons because the Coulomb force's range is larger. Protons and neutrons are joined together to form atomic nuclei by the nuclear force, also referred to as the strong interaction.
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