Why some massive stars become neutron stars and other black holes?

It depends on the mass of the star and how much of the star actually goes into the remnant.

Stars between 10 and 25 times the mass of the sun form neutron stars. Stars over 40 solar masses form black holes. Stars between 25 and 40 solar masses can form either depending on how much of the star is blown away during the supernova and how much falls back into the collapsing core.