Why did Zayd divorces Zaynab, may Allaah be pleased with them, as is clearly mentioned in Soorah Al-Ahzab, 37-38?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

It seems that you are inquiring about the reason why Zayd divorced his wife; the answer is in the hadith that Anas reported: “Zayd ibn Haarithah came to the Prophet complaining (about his wife's ill-treatment of him), so the Prophet told him, 'Fear Allaah and keep your wife...'” [Al-Bukhari]

Ibn Hajar while quoting Ibn al-‘Arabi, said, “The Prophet told Zayd to keep his wife to test him about his desire to keep her or not to keep her, but when Zayd informed him that he was resentful of her due to her arrogance and foul mouth, the Prophet gave him permission to divorce her...”

For more details on the wisdom why Zayd divorced her and the Prophet married her, please refer to fataawa 139996 and 177139.

Allaah knows best.