A nucleus becomes radioactive if it has more than 83 protons or if its neutron-proton ratio is less than 1:1 or greater than 1.5:1.
At close distances, a strong nuclear force exists between nucleons. This attractive force comes from the neutrons.
More protons in the nucleus need more neutrons to bind the nucleus together.
If there are too few protons, the nucleus will not be stable. It will decay to form a more stable nucleus.
As the nucleus gets bigger and more neutrons are added, the protons and neutrons are further apart.
The electrostatic repulsions between the protons get weaker.
The nuclear strong force operates over extremely short distances. It weakens even faster than the repulsions.
After a certain size, it can no longer hold the nucleus together. The nucleus decays to become more stable.
Beyond bismuth-209, the nucleons are so far apart that the neutrons can no longer hold them together. These nuclei are all radioactive.