The reason for that is simple: they've already proven through their words or actions that they can't be trusted. The first time around you had no way of knowing one way or the other, but now you do. Now you know that they can't be trusted, why in the world would you set yourself up for that kind of hurt and pain again?
That is why it's so difficult to overcome any kind of cheating in a relationship. You know the old saying: "fool me once shame on you, fool me twice shame on me", that's the whole point. Few people would take such a big risk of being hurt by letting someone who has already proven that they can't be trusted, into their lives again.
It's best to make darn sure you don't do anything to make your partner lose their faith in you in the first place. I know, a lot of people may be thinking that that is easier said than done, but not really.
While it's true that sometimes you find yourself becoming attracted to someone you see on a day to day basis, someone that you may not be able to avoid seeing like a co-worker. In a case like that you just have to keep as much distance as possible and not allow yourself to spend more time with them, or spend time with them outside of work, than is absolutely necessary.
A lot of times though, we do it to ourselves. We'll see someone out at a club when we're with our friends and one thing leads to another and before you know it you've done the very thing you promised you'd never do. In cases like that, you're just a jackass.
Sorry, rough I know, but true. Just because you're out on your own for a night with friends is no excuse to hook up with someone. If you can't stay faithful for one night then you need to really spend some serious time figuring out what is wrong with you because that's not normal behavior.
Some will consider themselves 'the man' or a 'playa' but really those are just ways to make themselves feel better about their own shortcomings. A real man would live up to his word and if he promised his girl that he would be faithful than that's what he'd do... period.
And I'm not picking on guys, ladies the same exact thing applies to you. If you really want trust in a relationship make sure that you don't do anything to betray that trust. It's very hard to risk being hurt and open up and trust someone, it's virtually impossible to do it a second time when that person proves to you that they can't be trusted.