I think the problem is two-fold. Firstly, there's the issue of social context. The very notion that society "always used to be x" is nonsensical from the start, because human society is in a constant state of change, and has existed in a myriad of different forms. Even limiting ourself to a single cultural heritage, the culture changes enormously over the years.
Within each period of a culture, various factors will influence the social dynamic, and who is or isn't empowered.
The second major complication is that our understanding of our cultural history is dictated by education, and western formal education was unfortunately established right smack in the middle of a period where "white, straight christian males" did indeed rule - namely from the Renaissance to the end of the 19th Century.
So much of western civilisation's understanding of our past is derived from this time period, and is clouded by that society's perceptions. However if you seek our primary sources for the Middle Ages or the Dark Ages or Classical Rome or Pre-Roman Britain you'll find quite a different story.
Because the Middle Ages was the period immediately before white man took control during the Renaissance, the Middle Ages are perhaps the most distorted of all periods. And unfortunately most fantasy writers set their works in a medieval setting, usually tainted by the Victorian notion of pastoralism.