Full Idea
The strong force holds quarks together within protons and neutrons, and residual effects of the strong force bind protons (whch repel one another) and neutrons together in nuclei.
Gist of Idea
The strong force binds quarks tight, and the nucleus more weakly
Source
New Scientist writers (Why the Universe Exists [2017], 02)
Book Reference
New Scientist writers: 'Why the Universe Exists' [John Murray 2017], p.27
A Reaction
So the force is much stronger between quarks (which can't escape), and only 'residual' in the nucleus, which must be why smashing nuclei open is fairly easy, but smashin protons open needs higher energies.