Why dont boys have periods?

From Wikipedia

Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive-age females of certain mammal species.

The answer to your question is the while a girl has a uterus, and therefore a uterine lining which can be shed every month, boys do not. The uterus is where the baby is grown before it is born.