Why is it the case that women are usually the reason for contributing the aneuploid gametes that leads to down syndrome?

Selection variables: If a woman generates an egg that contains 2 rather than 1 chromosome 21's, the addition of a 21 from the sperm will cause the trisomy 21 condition. Some of these miscarry & some deliver. When males generate a sperm that carries irregular information, there is speculation that the ones with bad information do not swim as well & thus can't pass the defect.