Well as a first cut protons have positive charge and like charges repel. This overwhelms any gravitational force between them.
Neutrons have no charge so no electrical force between them or between them and protons. The gravitational force is tiny.
Nuclear forces between proton proton proton neutron
2 answers
As a second cut you might ask why if they repel each other they stick together in the nucleus of an atom. I do not know if your course gets into strong force and weak force but here is a quick description.
https://www.livescience.com/48575-strong-force.html
https://www.livescience.com/48575-strong-force.html