Medieval law, not parliamentary rules, Stilly. Parliament merely codified an inheritance system that dates from the Norman conquest.

When her father took the throne, Princess Elizabeth (as she then was) became the "heir presumptive", to be pushed aside automatically if her parents ever had a male child.

For the record, Britain abolished the preference for male heirs only a very few years ago.

In the first scene of Shakespeare's "Henry V", the Archbishop of Canterbury and the Bishop of Ely have a learned discussion of Salic law, in which "mulieres ne succedant" [women don't inherit], and how it bars Henry from being King of France because his right to the throne comes from his mother. The same law was responsible for the civil war that broke out after Henry I died. He had named Matilda, his only surviving child as his heir, but the nobles who ran England preferred her cousin Stephen of Blois -- cue the Anarchy. Of course, this issue was topical in Shakespeare's own time because of, y'know, Good Queen Bess, and her obvious reluctance to marry and have children.