I believe it comes down to time and money. Photography equipment isn't cheap. And in order to really dedicate yourself to it, it will take a lot of free time. And not just to developed your craft, but also to promote your work. Traditionally, most of the people in this world with the money and free time to dedicate their lives to the art, and all arts in general, not just photography, are white males. Beyond that, is the connections one makes while being a part of the monied class. Gallery owners and their customers tend to have lots of disposable income, and people tend to associate themselves most often with others like them. So rich old white guys (generally not a demographic known to embrace racial and gender equality) with the money to spend on art, probably spend most of their free time around other rich old white guys. So the artists in their circles naturally tend to be rich white guys or friends and relatives of them. Art collectors outside those circles tend to want a good return on their investments, and buying art you know other rich people want is a more sound financial investment than buying art just because you like it. It's a self feeding cycle.
That being said, there is currently an ongoing explosion of diversity in the art realm. China is becoming a major player in the global financial market and bringing with them a meteoric rise in male Chinese artists. Women are becoming more powerful all over the western world, and so we're seeing more and more female artists emerge every day. So the old paradigm, while still in place, is being usurped. And while the majority of photographers are probably still white guys who have another source of income or means of financial stability outside of photography, that probably won't hold true for too much longer. And that's a good thing, because the art world has really stagnated since the middle part of the last century, and could use some fresh blood and ideas.