Final answer:
The slowing down of pulsars like the Crab Nebula pulsar is primarily due to the loss of rotational energy as they emit energy. This loss is consistent with the amount of energy observed from the surrounding nebula. Additionally, alignment of pulsar beams affects their detectability from Earth.
Step-by-step explanation:
The main reason for the observed slow-down of many pulsars, such as the Crab Nebula pulsar, is the loss of rotational energy. As a pulsar emits energy, it gradually loses rotational speed. This energy loss is significant due to the pulsar's incredibly high rotational speeds; even a slight reduction in speed results in the substantial loss of energy.
For the Crab Nebula pulsar, the rotational energy lost is observed as the emission of energy from the surrounding nebula. This energy transfer is sufficient to explain why the nebula shines with the amount of energy that we observe. Older pulsars which have slowed more significantly may emit less in visible and X-ray wavelengths and might only be detectable as radio pulsars.
In addition to slowing down due to energy loss, the visibility of pulsars is affected by the alignment of their beams with the Earth. Not all pulsars' beams align with Earth, which means many neutron stars may be undetected because their beams miss us entirely, akin to a lighthouse the beam of which does not reach certain areas.