The number of visible supernovae in the sky is relatively low compared to the vast number of stars in the universe. There are a few reasons why we don't see more supernovae:
Frequency: Supernovae are relatively rare events. They occur when massive stars exhaust their nuclear fuel and undergo a catastrophic explosion. Estimates suggest that, on average, a galaxy like the Milky Way experiences a supernova event once every few hundred years. Considering the vast distances between galaxies, the frequency of visible supernovae becomes even lower.
Distance: Supernovae that occur in other galaxies are often too far away to be seen with the naked eye. Even when a supernova occurs in a nearby galaxy, it can still be millions of light-years away, which means the light from the explosion takes a long time to reach us. By the time we observe it, the supernova may have already faded.
Visibility: Not all supernovae are equally bright. Some are incredibly luminous and can outshine their host galaxies for a short period of time, while others are dimmer and more challenging to detect. Only the brightest supernovae are typically visible to the naked eye from Earth.
Direction and Timing: Even if a supernova occurs in a nearby galaxy and is visible, its visibility depends on its location relative to Earth. If it happens in a direction where there is a lot of dust or gas that can obscure the light, it may not be observable. Moreover, the timing of the supernova is also a factor. It needs to occur when we are looking in the right direction and at the right time to spot it.
While the number of visible supernovae may seem relatively small considering the number of stars, astronomers actively monitor the sky and employ telescopes to detect and study supernovae in various wavelengths of light. This allows them to capture a more comprehensive view of these fascinating cosmic events.