``[Jesus said:] But I tell you that anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery.'' (Matthew 5:32)This is in line with God's original creation plan since he created only Eve for Adam.
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. Has not [the LORD] made them one? In flesh and spirit they are his. And why one? Because he was seeking godly offspring. So guard yourself in your spirit, and do not break faith with the wife of your youth. "I hate divorce," says the LORD God of Israel, "and I hate a man's covering himself with violence as well as with his garment," says the LORD Almighty. So guard yourself in your spirit, and do not break faith. (Malachi 2:14-16)
this contrast sharply with the attitude of Bible, where God said to Moses:
If a man marries a woman who becomes displeasing to him because he finds something indecent about her, and he writes her a certificate of divorce, gives it to her and sends her from his house, and if after she leaves his house she becomes the wife of another man, and her second husband dislikes her and writes her a certificate of divorce, gives it to her and sends her from his house, or if he dies, then her first husband, who divorced her, is not allowed to marry her again after she has been defiled. That would be detestable in the eyes of the LORD. Do not bring sin upon the land the LORD your God is giving you as an inheritance. (Deuteronomy 24:1-4)The reason for this prohibition of remarriage in the Bible is because of "defilement". Interestingly, the opposite (i.e. sexual relation with current husband) is required in Islam because remarriage can take place:
Narrated 'Aisha:
The wife of Rifa'a Al-Qurazi came to Allah's Apostle and said, "O Allah's Apostle! Rifa'a divorced me irrevocably. After him I married 'Abdur-Rahman bin Az-Zubair Al-Qurazi who proved to be impotent." Allah's Apostle said to her, "Perhaps you want to return to Rifa'a? Nay (you cannot return to Rifa'a) until you and 'Abdur-Rahman consummate your marriage." (Sahih Bukhari vol. 7, book 63; also no. 186, 187, 190, 238; book 72, no. 715; vol. 8, book 73, no. 107 ; vol. 3, book 48, no. 807; Malik's Muwatta, book 28, no. 28.7.18)