Because maybe they are bored with you. Maybe they tried to get attention from you at a certain point in your relationship when they needed it the most. Or maybe they just wanted to break up with you but didn't know how to do it?
I've been cheating on my wife for 34 years now but I think she just turns a blind eye to it. We do spend a lot of time apart because of my work, once even for a whole week. But the worst thing happened was when my wife became friends with my local mistress after they had met in the supermarket. I must say I did feel rather uncomfortable when she came over for a nice dinner my wife cooked. But I did manage to skip desert and quickly took off down the pub.
For some it's the thrill of forbidden lust. This is something I've never truly understood, for me, if I'm not happy with the one I'm with, I would move on. Unfaithfulness is the ultimate betrayal, usually unforgivable, it would be for me. Of course I would still love them, but no way ever would I remain with them.