Just my opinion here...
I think men are able to impress each other by having a lady on hand (the more attractive the lady, the more impressed the other men are.)
Women aren't able to do that as easily. We impress each other with clothes, hair styles, body type, etc. We don't need a man with us to impress other women, so we are more willing to cruise alone because it is more the cruise itself that defines us, not our cruise companion (or lack of one.)