Genes are Under Constant Attack

Every animal and plant genome is subject to a constant bombardment of deleterious mutations: a hailstorm of attrition. It is a bit like the moon's surface, which becomes increasingly pitted with craters due to the steady bombardment of meteorites. With rare exceptions, every time a gene concerned with an eye, for example, is hit by a marauding mutation, the eye becomes a little less functional, a little less capable of seeing, a little less worthy of the name of eye. In an animal that lives in the light and uses the sense of sight, such deleterious mutations (the majority) are swiftly removed from the gene pool by natural selection.

But in total darkness the deleterious mutations that bombard the genes for making eyes are not penalized. Vision is impossible anyway. The eye of a cave salamander is like the moon, pitted with mutational craters that are never removed. The eye of a daylight-dwelling salamander is like the Earth, hit by mutations at the same rate as cave-dwellers' eyes, but with each deleterious mutation (crater) being cleaned off by natural selection (erosion). Of course, the story of the cavedweller's eye isn't only a negative one: positive selection comes in too, to favour the growth of protective skin over the vulnerable sockets of the optically deteriorating eyes.

Constantly bombarded by things that mutate them so that the moment the expression is no longer used, it becomes ruined, thus we have blind animals living in caves.