The Maharal of Prague writes (Tiferet Yisrael, page 17) that women are closer to spiritual perfection than men, and subsequently they do not have the need for mitzvot in the way that men do. That is one of the reasons that women are exempt from time bound positive mitzvot, but there is more to it than that. Mitzvot are an external expression of our connection to God. Men, being more physical and more external, need the mitzvot to physically draw them upwards towards God. Women do not need them in the same way and their "Mitzvah load" is considerably less than for men as it reflects their inner spiritual connection. The concept of a Brit is to have a continual and external sign that we are the Chosen Nation of God. Women do not need one as their connection is more innate and more personal.
