It is because He knows that you have broken your promise to the wife you married when you were young. She was your partner and you broke your vows before God. Yet He made you one with her. So that you might have holy children. Divorce is a grave offense against the natural law.
Divorce is immoral because it introduces disorder into the family and society.
Currently in America half of all new marriages end in divorce. The American Catholic Church is complicit in this evil by granting wholesale annulments. The Catechism says divorce traumatizes children. One need not look far to see the consequences of that trauma. Abortion and teenage pregnancy are higher in divorced homes.