There are many possible reasons why some amount of deleterious alleles are maintained in the population. One of which is the mutation-selection-drift balance. In short: Because mutations always occur, there is continually an input of deleterious mutations in populations genome resulting in some fitness decay (called mutation load). While mutations create this polymorphism, processes of selection and genetic drift cause a decrease in polymorphism. At equilibrium there is a balance between mutations, selection and drift called the mutation-selection-drift balance. You may also want to consider various population structure related concepts such as migration between patches. Haldane first explored this mutation load theoretically and estimated that the fitness of an existing individual in a population of perfect (deleteterious-mutations-free) individual is about 80% lower, which is huge! Such mechanism can explain various disease found in a population.

Also, for the question of disease appearing late in life (which is often the case of eyesight related disease), it is important to realize that those genes which are expressed late in life undergoes a lower selection resulting in a higher mutation-selection-drift balance. See [this post][1] for more info.

As pointed out by @potterbond007 modern medicine allows to diminish the deleterious effect of some mutations. For example, one that has [clubfoot][2] may be operated and is likely to survive and reproduce and therefore propagating its deleterious alleles. This would be different in some past. Therefore, medicine diminish the selection pressure on deleterious alleles. However, modern medicine probably has a very low effect on our genomes because it is so modern. 100 years ago we could not operate clubfoot and 500 years ago (almost) nobody had glasses to correct for eyesight deficiency. We may go a bit further in the past and also consider that social interactions such as helping weak individuals in a tribe may also have decrease the selection pressure against deleterious alleles. Of course, if selection is diminished the mutation-selcetion-drift balance is increased and deleterious alleles are more common and mutation load is greater.

Finally, if it seems to you that eyesight-related diseases are more common and more important in terms of their effects on fitness than diseases on other traits then, you may want to seek into some physiological and developmental explanations that may tell you why eyes are so likely to be impacted by genetic and/or environmental factors.



  [1]: http://biology.stackexchange.com/questions/17077/why-does-evolution-not-make-our-life-longer/
  [2]: http://en.wikipedia.org/wiki/Club_foot