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.
God made husbands and wives to become one body and one spirit for his purpose -- so they would have children who are true to God. So be careful, and do not break your promise to the wife you married when you were young.
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.