Why don't nuclear protons, with formal positive charges, electrostatically repel each other?
They do, but they are constrained by a stronger force.
The strong nuclear force, while short range, operates within the nucleus between protons and neutrons. At this level, it is stronger (demonstrably) than the electromagnetic force that would repel positively charged protons. For a more sophisticated answer, you are going to have to find a particle physicist, at least someone better informed than I am.