Why does inbreeding increase the chances of genetic disorders?

because with rare recessive alleles, you are more likely to find multiple heterozygotes in the same family. Thus crossing two people from the same family you are more likely to cross two heterozygotes and thus have a chance of producing the double recessive. On the other hand if the heterozygote mates in the general population, the likelihood of finding a mate who is heterozygous for the trait is slim to none, meaning offspring will be 50% homozygous dominant and 50% heterozygous, with no offspring expressing the recessive trait.