Heavier nuclei tend to exhibit a larger N/Z ratio because

In a nucleus, protons repel each other due to the electrostatic Coulomb force, which is a long-range force. This means every proton repels every other proton in the nucleus, and the total repulsion increases rapidly as the number of protons increases.

The strong nuclear force, which binds nucleons together, is a short-range force. A nucleon only interacts strongly with its nearest neighbors.

To maintain stability against the growing Coulomb repulsion in heavier nuclei, a greater proportion of neutrons is required. Neutrons contribute to the attractive strong nuclear force but do not exert any electrostatic repulsion.

Therefore, the larger $N/Z$ ratio in heavier nuclei is because neutrons do not exert electrical repulsion and Coulomb forces act over a longer range than nuclear forces.