I think you're right that there isn't much difference.You should have just asked what leads people to cheat. Men and women really arent that different. So, the exact same things that lead to men cheating. Unhappiness in their relationship, feelings of neglect(including but not limited to lack of sex), being taken for granted, etc, all while the attraction to and possibility of a different guy fills the needs she feels are neglected. I.E. She doesn't feel like you appreciate/are attracted to her while the other guy does make her feel that way.
Or, also just as with guys, sometimes they're just incapable of monogamy. Though I do feel like that's more common with men than women in my, admittedly biased, opinion.
And as to the reason? Selfishness is the reason.