Because really: This crisis was all about Joseph, wasn’t it?
Mary was just a girl. Even by today’s standards she wasn’t an adult who could be trusted to make her own decisions. She was the responsibility of her father, who was giving that responsibility to Joseph. Whom we can understand being a little miffed that she wasn’t quite the untouched vessel for his children that he was promised.
Fortunately, he was an enlightened and sympathetic man, who knew just what to do.