King James Version
Yet ye say, Wherefore? Because the LORD hath been witness between thee and the wife of thy youth, against whom thou hast dealt treacherously: yet is she thy companion, and the wife of thy covenant.English Standard Version
But you say, "Why does he not?" Because the LORD was witness between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant.New International Version
You ask, "Why?" It is because the LORD is acting as the witness between you and the wife of your youth, because you have broken faith with her, though she is your partner, the wife of your marriage covenant.