There are a number of possible explanations: 1) Disabilities such as Down's syndrome, cerebral palsy, etc. may not have been Evolutionarily removed because they may not have had a negative impact on an individual's ability to survive and reproduce. 2) Disabilities such as Down's syndrome, cerebral palsy, etc. may not have been Evolutionarily removed because the individuals who had them may have had other qualities or traits that offset the disadvantage of the disability. 3) Disabilities such as Down's syndrome, cerebral palsy, etc. may not have been Evolutionarily removed because the individuals who had them may have been able to find mates who were also willing to mate with them despite their disabilities.