You ask, “Why?” It is because the Lord sees how you treated the wife you married when you were young. You broke your promise to her, even though she was your partner and you had an agreement with her.
The Lord God of Israel says, “I hate divorce. And I hate people who do cruel things as easily as they put on clothes,” says the Lord All-Powerful. So be careful. And do not break your trust.
Marriage should be honored by everyone, and husband and wife should keep their marriage pure. God will judge as guilty those who take part in sexual sins.