Bukhari

Because Allah the Exalted says (in Surah Al-Baqarah) «فلا تعضلوهن‏» "When you divorce women and they have fulfilled their term, do not prevent them from remarrying their husbands." This includes both previously married and virgin women. And Allah the Exalted has said in the same Surah «ولا تنكحوا المشركين حتى يؤمنوا‏» "And do not marry polytheistic men [to your women] until they believe." And in Surah An-Nur He said «وأنكحوا الأيامى منكم‏» "Marry off those among you who are single."

Narrated 'Urwa bin Az-Zubair: 'Aishah, the wife of the Prophet (saws) told him that there were four types of marriage during Pre-Islamic period of Ignorance. One type was similar to that of the present day i.e. a man used to ask somebody else for the hand of a girl under his guardianship or for his daughter's hand, and give her Mahr and then marry her. The second type was that a man would say to his wife after she had become clean from her period. "Send for so-and-so and have sexual intercourse with him." Her husband would then keep awy from her and would never sleep with her till she got pregnant from the other man with whom she was sleeping. When her pregnancy became evident, he husband would sleep with her if he wished. Her husband did so (i.e. let his wife sleep with some other man) so that he might have a child of noble breed. Such marriage was called as Al-Istibda'. Another type of marriage was that a group of less than ten men would assemble and enter upon a woman, and all of them would have sexual relation with her. If she became pregnant and delivered a child and some days had passed after delivery, she would sent for all of them and none of them would refuse to come, and when they all gathered before her, she would say to them, "You (all) know waht you have done, and now I have given birth to a child. So, it is your child so-and-so!" naming whoever she liked, and her child would follow him and he could not refuse to take him. The fourth type of marriage was that many people would enter upon a lady and she would never refuse anyone who came to her. Those were the prostitutes who used to fix flags at their doors as sign, and he who would wished, could have sexual intercourse with them. If anyone of them got pregnant and delivered a child, then all those men would be gathered for her and they would call the Qa'if (persons skilled in recognizing the likeness of a child to his father) to them and would let the child follow the man (whom they recognized as his father) and she would let him adhere to him and be called his son. The man would not refuse all that. But when Muhammad (saws) was sent with the Truth, he abolished all the types of marriages observed in pre-Islamic period of Ignorance except the type of marriage the people recognize today.