Why SHOULD women mature earlier than men? I've never heard this question phrased quite that way. But I suppose because women give birth. And women are still primarily responsible for raising children. For this reason, women need to have the social skills and education to instill good values and morals in young children. Every child needs a mentor to look up to and, at least in the early years, this is the mother.
To answer ptosis' question, there are mature men out there. It depends on where the woman is looking. But bad habits die hard. And women (and men) keep making the same mistakes over and over.