The fact that women mature earlier is one element, the fact that they seek qualities that men acquire later (stability, resources) is another.

But I think that what makes the situation very sad is that our western society has decided to close its eyes to the intrinsic value of women's virtue in the eyes of men. When a young man can't find an interested partner during his younger years and then he is supposed to either fall back on the women over 30 who are left or play on his newly acquired skills as a thirty year old to seduce a woman 10 years younger. In the end, everyone is sad, there are fewer children, society is less virtuous, women change partners throughout their lives, while men wonder if there is any marryable woman left around them.