My understanding was that for a very long time, across the world, societies held to a system of primogeniture and favoured the male children. How did women like Elizabeth I and Catherine the Great come to the head of their respective monarchies?
To start with this thread from two days ago about Mary (Liz 1's predecessor) has some good answers.
Well first of all, Elizabeth I followed her sister, Mary I. And pretty much, to put it as simply as possible, when their older brother, Edward VI, died without any heirs there was a power struggle. Lady Jane Grey who was their cousin and a Great-Granddaughter of Henry the VII was named by Edward as his successor. However, Mary, with the love of many of the people and the backing of some powerful noble families was able to take the crown back. Mary would also die childless leaving Elizabeth the only obvious affair. It's also important to keep in mind Henry VIII had quite a few relatives who had ties to the throne killed during his reign so by the time it came to Mary and Elizabeth there weren't many favorable alternatives.