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.