Why are pulsars not always associated with supernova remnants?
Pulsars may or may not be inside supernova remnants.
It is widely accepted that pulsars are fast spinning, highly magnetised neutron stars.
Neutron stars are formed in the late stages of stars over 8 solar masses. As the star ages fusion reactions created heavier and heavier elements until the core is mainly iron. No further fusion reactions can take place as fusion reactions involving elements iron and heavier consume energy rather than release energy.
Once fusion reactions stop, the core collapses under gravity. The core collapses and heats up to form a neutron star. Te radiation released by the collapse blows the rest of the star away to form a Type II supernova.
It is therefore believed that all neutron stars, and hence all pulsars are associated with a supernova explosion.
However, during the formation of the neutron star it needs to gain angular momentum to be a pulsar. It can also gain linear momentum. So, although many pulsars are inside supernova remnants, also known as nebulae, others are not because over time they have drifted away from the location they were formed and are no longer inside the nebula.