A recessive allele only shows up in the phenotype (is only manifested physically) if there is no paired dominant allele to cover it up. Natural selection against a recessive allele can only act when the recessive allele shows up in the phenotype. This means that there are a lot of organisms carrying an invisible recessive allele around. As long as the recessive is invisible (paired with a dominant allele) natural selection can't find it and can't do its job.