You ask, “Why?” It’s because the Lord is holding you responsible. He watches how you treat the wife you married when you were young. You have been unfaithful to her. You did it even though she’s your partner. You promised to stay married to her. And the Lord was a witness to it.
“Suppose a man hates and divorces his wife,” says the Lord God of Israel. “Then he is harming the one he should protect,” says the Lord who rules over all. So be careful. And don’t be unfaithful.
For example, by law a married woman remains married as long as her husband lives. But suppose her husband dies. Then the law that joins her to him no longer applies.
All of you should honor marriage. You should keep the marriage bed pure. God will judge the person who commits adultery. He will judge everyone who commits sexual sins.