I've been thinking about this question for a few days now, in conjunction I've also been thinking about masculine vs. feminine and how we have to develop both aspects within ourselves. So my latest conclusion is that poly requires that women develop the masculine aspect of autonomy, freedom and independence - normally a male characteristic; and men develop the feminine aspect of expressing emotion and emotional intimacy. Both are a struggle for either side and go against the grain of society. I think many men find the emotional investment of intimate relationships to be a big deal and a lot of work and can feel very vulnerable in the undertaking of that investment so are wary of it. Women on the other hand find that this aspect of themselves comes fairly naturally.
disclaimer: the question itself is a basic generalization so the answers swerve into generalizations. I think we all realize the danger of generalizing but understand the purpose in this context.