Here's why:

* Extreme Density: Neutron stars are formed from the collapsed cores of massive stars after a supernova explosion. The immense gravity crushes the core, squeezing protons and electrons together to form neutrons. This results in a density that is incredibly high, even exceeding that of an atomic nucleus.

* Tiny Size: Despite containing the mass of a star, a neutron star is only about 10-20 kilometers in diameter. This makes them incredibly compact, further contributing to their extreme density.

* Exotic Matter: The conditions inside a neutron star are so extreme that they create exotic forms of matter, like superfluids and superconductors.

While black holes are even denser, they are not technically considered a state of matter. They are regions of spacetime where gravity is so strong that nothing, not even light, can escape.