Because when it is a boyfriend/girlfriend kind of situation, there isn't as much at stake (usually), as there is in a marriage. Lately though, lines have really begun to become blurred there. It used to be that when you were married, you generally had a family, and a home, and other joint assets and accounts, and you promise to stick together no matter what. Through good times and bad etc..... So trying to get over an affair just makes practical sense, and boyfriends/girlfriends were just that. They had no ties to one another. But any more people start buying houses together and having kids before they get married, or never get married at all.
I agree cheating is cheating, but sometimes it depends on what else you have to lose. If you have a family and all that goes with it, I think you should try to give it another shot, before ripping all that apart, married or not, but if it's just the two of you and there is cheating, what is the point of trying to save something that is bad before you have a deeper commitment? If they cheat when it's just the two of you, you know it will be much more enticing when there's more responsibilities later on.