Why is huntingtons chorea not selected against to a great extent in the normal evoluntionary process?

That's an excellent point. If a particular disease or disorder doesn't appear until well after reproductive maturity, it will be impossible for natural selection to eliminate it. This is why many human ailments begin to appear after the age of 40. Also, some disorders arise from mutations that occur within the cells of parents and just appear without natural selection having the chance to eliminate them. Also, certain disorders are only expressed in the zygous state (double recessive) and you can have carriers carrying around "bad" genes that won't be expressed until they are paired up with another "bad" gene during ual reproduction. Natural selection cannot eliminate a single gene that is not being expressed in the phenotype of the host. Therefore, there are many reasons why natural selection cannot eliminate all disorders.