Duchenne muscular dystrophy is a genetic disorder that arises from a recessive X-linked allele. Why is this disorder almost always restricted to males?

Since Duchenne muscular dystrophy is X-linked recessive, it requires all of the X chromosomes in a human to take effect. This is what makes it recessive.

Since females have two X chromosomes, both of their X chromosomes must have the allele for Duchenne muscular dystrophy in order to be an active disease.

However, since males only have one X chromosome, they only need to have the allele for Duchenne muscular dystrophy on that single chromosome in order to be an active disease.

It is much more likely to get one affected X chromosome than it is to get two affected X chromosomes, therefore this disease is almost always restricted to males.