Imam al-Shafi'i (may Allah have mercy on him) said: "Because Allah, the Mighty and Majestic, made it unlawful only on account of the sanctity of the lawful (relationship), and the unlawful (relationship) is the opposite of the lawful."
He said: "And our view has been narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / Q13960