I've heard a few theories on this... First off, better nutrition. It's a fact that improved nutrition in just the last few hundred years has led to Americans being taller than their colonial ancestors. It's thought by some that better nutrition allows girls to mature faster.
Antoher theory is due to eating more meat that has been treated with growth hormones, which causes faster maturation.
Mark
There's also evidence to the effect that, statistically speaking, girls who grow up in a household with their biological fathers reach puberty later than those who grow up with a single mother or stepfather.
Probably it's a mix of a lot of things, with "more fat in the diet, and being heavier" being my guess as to the leading factor.