Why are females last in line in order of hereditary succession for the throne of the united kingdom?

I'm American and not even up on the latest American laws, but I thought a few years ago, maybe 10 or 15, that I heard that there was some kind of change in British law, that would not effect Princess Anne, but from here on that the heir would be the oldest regardless of if they were male or female. Maybe I'm wrong, or maybe it did not p Parliament. In other words if William married and had a daughter and then a son, the daughter would be heir to the throne, not her younger brother, but I don't know.