Ok, this is a good one.
Let's see - for what do we need models? For advertising.
Why do we need advertising? For selling goods.
What goods are sold by using advertisments with models? Clothes, mobile phones, accesorizes, some times food or some other stuff.
Who spends more money on buying these goods - men or women? Of course, women.
So, demand on female models is much higher - women would like to see how clothes\accesorize will look on them. Also, female models are really popular among men.
There is one more thing - fashion. Men are not so interested in fashion, so usually modeliers create clothes and stuff for women. Who will show these new collections to the world and earn millions? Female models.
That seems to be all for today. :)