Answer:
The answer to this question has several factors, but probably the most important lies in the difference in eye structure and size of the different structures of the eyes, especially those specific for light detection and perception, and also, due to the difference in the amount of two types of cells in the eye, the Rods, and the Cones, which are responsible for light reception and color reception.
Most animals, especially nocturnal animals, or those who also have the ability to see at night and day, like cats, or dogs, have several factors that make them better at night vision than humans. The first factor is the structure of their eyes, and the organs that play a role in light perception. These structures are better developed, of bigger size, and capable of extracting light out of the lowest light densities, whereas the human eye is incapable of that, which is why human eyesight at night is poor. A second really important factor is the difference in the availability of one of the two types of eye cells, which are responsible for light and color perception: the Rods and the Cones. In these animals with nocturnal capability, the Rods are the most common, and thus, they can perceive more light, even when there is very little of it, from the environment. This is also why these animals cannot see in color at night, because they have less Cones, and more Rods.
Finally, there is also a small membrane in the eyes of certain animals, especially nocturnal onces, called the tapetum lucidum, which allows reflection of the light, and even further capturing of it.
These are all reasons why certain vertebrates are much better at night vision than humans.
