Why men leave their mistress?

NO. They usually do NOT leave their wives. I think it's something like 3% of women who actually get to have a relationship with a man who leaves his wife. Men will often keep 'the other woman' stringing along for YEARS, lying and making promises that they know full well they will never keep. They want to have their cake and eat it too, so they will keep a mistress along for as long as she will put up with it.