Because for women, looks aren't as heavily emphasized. A less physically attractive male can make up for his looks with such things that can matter just as much as more more than looks... like charisma, amazing personality, admirable intelligence, success in career, money/financial stability, working out, power in the community, being part of a culture the girl is interested in, heroic/noble deeds like firefighter work or serious charity work, being a good lover, etc. Plus, since looks matter more for women, maybe she feels more attractive next to a male less attractive? I don't know.
Be really, really, really nice. That's all I can really say. I know a bunch of not-so-good-looking guys, but because they're just so lovely and nice and caring and considerate, other girls do end up developing crushes on them.