The white dwarf is not as massive, is larger, and less dense than the neutron star. There are more white dwarf stars than neutron stars. The main reason is that there are more low mass stars than high mass stars to begin with. Another reason is that all low mass stars end up as white dwarfs. High mass stars can end up as neutron stars or black holes.
Supernova remnants are very small.
Therefore, they are difficult to see using visible wavelengths. Neutron stars tend to have rapid rotation and very strong magnetic fields . These conditions can result in an object called a pulsar. Pulsars are very easy to detect with radio waves because of the rapid fluctuations. A rapidly changing object is easier to detect than a stagnant object, much as a deer is more visible in the woods if it is moving.