I coulde easily flip this question to ask why do men cheat on their wives?
The reality is that both men and women cheat for the same reasons, which include incompatibility, boredom, need for excitement, curiosity, a desire for adventure, the thrill of breaking the rules, the knowledge that their partner is already doing it........ and so on.
I've said this before, but I'll repeat myself: Look at yourself first, ask yourself why your partner might cheat on you. Are you the same person that you were when you married? If you've let yourself go, got sloppy in manner and dress, got fat and lazy, too hooked on watching TV instead of spending time together doing things. Has your sex life got staid and repetitive, is your partner bored with doing the same old things, or maybe you're not bothering to do at all.
All of this applies equally to men and women. So if I've described you or your partner, don't be surprised if someone is looking elsewhere.