There might be a few different reason for this, although I don't believe any of those reasons are sexist or premeditated. First of all, Golding based his characters on a group of boys from a private British school. Note, schools for boys were schools for boys. Private schools at the time were not co-ed, thus, girls would not have been there..... period. Also, little boys have always been credited with rough behavior. Men went to war..... not women, except as nurses. The "Lord of the Flies" is a product of its time, and I don't believe the lack of girls has anything to do with savagery or a quest for power.