I wonder why the neutron stars did not collapse into black holes.
Not enough mass.
If you look at Wikipedia the star that collapses into a neutron star is not massive enough. As this star collapses the protons and electrons merge to become neutrons. Based on the rules of quantum mechanics the force of gravity is not enough to overcome the tendency of the neutrons not to want to crunch into each other; neutron degeneracy pressure. In a black hole there is enough mass in the original star to overcome this pressure.
Overall in a star gravity wanting to crunch the object is working against another force. In our sun the heat generated by fusion stops gravity. When out sun runs out of fuel the star will collapse until the repulse force of the electrons stops the collapse forming a white dwarf. As I mention in a neutron star the repulsive force of the neutrons stops gravity.