Imam al-Shafi'i (may Allah have mercy on him) said: "Because he now has no wife, and no waiting period (iddah) is binding on him."
And he argued that the marital bond has ended from the fact that its rulings, namely ila', zihar, li'an, inheritance and so on, come to an end.
He said: "This is the view of al-Qasim bin Muhammad, Salim bin Abdullah, Urwah (may Allah have mercy on them) and most of the people of knowledge of the Abode of the Sunnah (Madinah), and Allah, the Mighty and Majestic, has forbidden."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q13850
Urwah bin al-Zubayr and al-Qasim bin Muhammad (may Allah have mercy on them both) both said, concerning a man who has four wives and divorces one of them irrevocably, that he may marry whenever he wishes and need not wait for her waiting period to pass.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13850
Qatadah (may Allah have mercy on him) narrates from Sa'id bin al-Musayyib (may Allah have mercy on him) concerning a man who had four wives and divorced one of them. He said: "If he wishes, he may marry a fifth during her waiting period." He said the same concerning two sisters.
(b) Qatadah (may Allah have mercy on him) narrates from Sa'id bin al-Musayyib (may Allah have mercy on him) concerning a woman who has been irrevocably divorced.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13851