I do not have all the answers of why this happens, but I will attempt to respond based on a little reading, and because this is an area that has fascinated me for many years. There are several reasons for congenital blindness including an inherited condition such as pressure in the eyeballs that leads to blindess (glaucoma), there maybe some defect in the eye (so they cannot see) or in the brain (so it does not receive or transmit information to the eye). Congenital blindness could also be caused by an infection during pregnancy or injuries during child birth. Sometimes, people develop blindness as they age, due to preexisting conditions such as cataract where the lens of the eye becomes progressively opaque. Can you think of other factors that can contribute to blindness in later life?