It doesn't really. I'm sure the phrase came from people saying "so much win" or other nonsense when topless photos were uncovered, and now they probably repeat it because it sounds less sleazy than the alternative words.
Nah you're all mistaken the origin is actually that getting sexy pictures of girls is like a game and topless pictures is seen as the ultimate victory. For example, a person "wins" if they find the gem of a naked picture of a popular cosplayer. It comes from topless pictures literally being a symbol of fruits of labor.