Men want to be wanted, and women generally don't want to be with a lower-status man than themselves.

What proportion of women tend to date men who earn less than them, are shorter than them, have a lesser degree than them? Not a very big one, and men know it, so they steer clear of those women.

Similarly, most men prefer to date women shorter than them, who earn less than them, etc. Why? I'd guess because most men like to feel like they're providers and protectors, and most women like to feel provided and protected for by their partner.