A couple of reasons; first, women (& girls) are innately more in touch with each other on a familiar level. That intergender affection is commonplace and encouraged socially, so it's a small step going from a familiar affection to sexual affection. Have a read of
this article for the biological breakdown of how women process this neurologically.
From a male perspective, heterosexuals and homosexuals are far more isolated in their sexual identities. I wish I had the link handy, but somewhere on SS was posted a link to brain studies done in which heterosexual and homosexual males were exposed to a variety of sexual images. Not surprisingly each were only aroused by their respective sexual preferences; men with women and women with women aroused heterosexuals, men with men for homosexuals. However, when women were exposed to the same imagery they became aroused by EVERY image to varying degrees.
Obviously from a social standpoint it's far more acceptable for women to be sexual with other women than for men to be so with men. Bisexual behavior amongst women is reinforced to a far greater degree than in men in western culture. In fact for the masculine it's almost an absolute; women can be bisexual, but men are gay from the moment they engage in their first homosexual act, from a social perspective. Masculinity is far more rigid than femininity in this respect.
That's not to say there aren't ways for men to express affection for one another, it's just done so in the form of respect or giving each other sh!t. Genuine masculine affection is almost always rooted in competition. In fact it's just this "ribbing" and the level of acceptance of it that sets a gauge for how understanding a man is with regards to masculinity. Giving another guy sh!t in jest is a form of affection - I wouldn't ƒuck with you if I didn't like you.
