Here's why:

* Main Sequence Stars: These are stars that are fusing hydrogen into helium in their core. This process is what gives them their energy and luminosity.

* Neutron Stars: These are the extremely dense remnants of massive stars that have exploded as supernovae. They are primarily composed of neutrons and do not undergo nuclear fusion.

In summary: Neutron stars are not actively fusing hydrogen, which is the defining characteristic of main sequence stars. They are completely different types of celestial objects with different origins and properties.