Why did the Prophet, may Allah bless him and grant him peace, not marry anyone other than Lady Khadijah, may Allah be pleased with her, during her lifetime?

The Prophet (peace be upon him) did not marry anyone else while Khadijah was alive, until she passed away, out of honor for her and due to her great status with him. This is evidence of her special virtue, as she sufficed him from others, and preserved her heart from jealousy and the annoyance of co-wives. There is no disagreement among scholars regarding this.

Ibn Hajar said: "The Prophet (peace be upon him) lived 38 years after his marriage to Khadijah. Khadijah alone accounted for 25 of these years, which is about two-thirds."

Ibn Kathir said: "He did not marry anyone else during her lifetime due to her majesty and great position with him."

And it is obligatory for a believer to glorify and venerate their Prophet (peace be upon him), whether they know the reason for the wisdom behind it or not, as in His (Allah's) saying: (Indeed, We have sent you as a witness and a bringer of good tidings and a warner (8) That you [believers] may believe in Allah and His Messenger and support Him and honor Him and exalt Him morning and afternoon (9)).

Sheikh Ibn Sa'di said: "Meaning: you glorify and venerate the Messenger (peace be upon him) and fulfill his rights."

These matters are specific instances that do not entail action or religious obligation. What suffices the believer is what has become apparent to him of the objectives of the Sacred Law and aspects of wisdom. There is no need for him to meticulously inquire into what does not result in action and does not have significant benefit.