Campbell was looking back at particular types of stories, myths, that emerged from patriarchal cultures that often associated the feminine with mystery, magic, fertility, and the like. Because the myths take place in that context, the feminine aspects take on a great symbolic significance in the journey that is separate from that of the hero. I don't think that means the hero's journey cannot be taken by a woman--it certainly can. But the fact that women take on a symbolic meaning separate and apart from the 'hero' makes some people interpret it exclusively, as though Campbell was relegating women to that symbolic role in the cycle. I don't think he was--he was just pointing out how the commonalities play out in the mythology.