Because you put up with it. Seriously! Don't want to offend you, but it's true, no?

Why haven't you confronted him about his cheating again with this woman? It seems more like you want to know why he did it rather than stop him from doing this. How can you even want to marry a man like this? It won't stop after the two of you walk down the aisle... the problem will still be there until you take a stand. First of all, you need to tell him that this is absolutely unacceptable. Then force him to go to counseling with you. Just taking his word for it that he won't cheat anymore didn't work before, and it won't work now. And you absolutely can not marry this guy!! Don't even set a date or start making wedding plans. In fact, i think you should give him back the ring, and insist that if the two of you work things out, he can propose to you again. Sweety, divorcing him is alot harder than breaking up with him. And if this guy doesn't change, then you need to tell him to hit the road.

Hey, i came across a post made by LCD an Feb 6th called 10 Ways To Marry The Wrong Person. You should read it if you haven't already.

10 Ways To Marry The Wrong Person

Hey, sweetheart. I hate to break it to you, but here's the simple protocol. He is a cheat. You can't change that. I think what you're looking for, ultimately, is advice on how to make him stop cheating on you and turn him into the man you deserve. You're letting the guy try to get closer closer to you and more deeply involved with you after you know what he's already about. That's why I say that.
If he really wants to be with you and is serious about changing, there may be hope, but 99.9% of the men who do this aren't sorry about the pain they caused you, they're sorry they got caught and they do have some very creative ways to fool you and themselves into believing otherwise, but I can give you a clue as to what the real indication is that he's not going to change. If you 'forgive' him, but he has to earn back your trust and can't rush you, what a man who is just looking to get off the hook will do is keep changing the subject, being defensive, and pestering you to 'start all over again'. "If you've forgiven me, why to you keep bringing it up?" Or he'll try to say that it was something you did to make him cheat. Nothing short of someone holding a gun to his head is acceptable. You must understand. You have to accept what he really is, so you can start healing yourself. You also can't want to rush to 'good feeling' portion of this either, because you're repeatedly hood winking yourself with a quick fix of affection. That quick fix is only good for the moment, not for the reltionship. Good luck and bless you.