PCGamer reports that the IeSF, a global e-sports organization headquartered in South Korea, stipulates male-only participants "in accordance with international sports authorities, as part of our effort to promote e-sports as legitimate sports."
Assembly Summer 2014 administrator Markus Koskivirta says that the event's other tourneys are open to all genders. "We would also like to point out that the Finnish eSports Federation is currently lobbying for the equal rights of male and female players in the IeSF tournaments," he said.
[Commenters have pointed out that women are barred from four games in this tournament, including Hearthstone and Dota, while male players are barred from two, including Tekken Tag, in the IeSF's attempt to create gendered brackets for e-sports. Both genders can compete in StarCraft II, but only in gender-segregated divisions. Thanks to CrowingOne especially for the extra details.]