Why is there a 50% chance of black or yellow if black is dominant and yellow is recessive?

Because in the specific example used, one parent has two recessive alleles and the other parent has one dominant allele and one recessive allele. If you look at the resulting square, it's clear why the possible outcomes are 50/50: there's a 50% chance that the offspring will inherit recessive alleles from both the father and the mother. If both parents had one dominant allele and one recessive allele, then there would only be one possible recessive-recessive combination, and the chance of yellow in the offspring would be only 25%.