I would say it's most likely caused in part by larger variability in skill level (or components of skill like reflexes and hand-eye coordination) among men than among women. Competitiveness and willingness to accept the costs of being a hardcore gamer probably also play a part.
Even if the average skill level (or competitiveness, or dedication to playing a game for 5000 hours, and so on) is the same for both men and women, a higher variability among men means that there will be more men at the highest tiers.
If you're interested in learning more about the statistics behind what I'm talking about, I suggest reading this article:
http://nsm.uh.edu/~dgraur/niv/TheMostDangerousEquation.pdf
It includes a discussion of the differing variability between men and women, though not specifically relating to gaming.
Even if the average skill level (or competitiveness, or dedication to playing a game for 5000 hours, and so on) is the same for both men and women, a higher variability among men means that there will be more men at the highest tiers.
If you're interested in learning more about the statistics behind what I'm talking about, I suggest reading this article:
http://nsm.uh.edu/~dgraur/niv/TheMostDangerousEquation.pdf
It includes a discussion of the differing variability between men and women, though not specifically relating to gaming.