Why did the Messenger of Allah (peace and blessings be upon him) marry Maryam the Copt while he had wives at that time?

The Shaykh : Why did the Messenger of Allah (peace and blessings be upon him) marry Maryam the Copt while he had wives at that time? And is there a Shari'ah reason or explanation for the Messenger of Allah (peace and blessings be upon him) not freeing her and marrying her?
This question actually contains two questions:
The first question: Why did the Messenger of Allah (peace be upon him) marry Maryam the Copt while he had wives at that time?
I say with complete frankness: I do not know why he married her. I only know for certain that he did what Allah permitted him to do. Allah permitted him to marry as many women as he wished. As for why he married that specific woman, I do not know the secret behind it. But I also know for certain that he married her for her benefit, not to harm her, and to protect her chastity, not to lead her into sin, and for other benefits that cannot be fully comprehended. Some Islamic writers have writings that are not objectionable in explaining or philosophizing about the Messenger of Allah's (peace be upon him) marriage to many women. They explain and justify why he married each one. For example, he married Umm Salamah because her husband had died, leaving her with young children, so he married her to be their guardian, to raise them, educate them, and provide for them, and so on. He married A'ishah because she was the daughter of his companion in the cave. He married Hafzah, and so on. They find valid reasons for each wife for which the Messenger of Allah (peace be upon him) married her. But the matter is broader than that, such that a person cannot encompass the thoughts and the wisdoms for which the Messenger of Allah (peace be upon him) married the women he married.
As for the second part of the question, which is: Is there a Shari'ah reason for the Messenger of Allah (peace and blessings be upon him) not freeing her and marrying her?
It is as if the questioner is saying: Why did the Messenger of Allah (peace and blessings be upon him) not treat Maryam the Copt as he treated his wife Safiyyah? It is established that he (peace be upon him) freed Safiyyah and made her freedom her mahr (bridal gift). This is in "Sahih al-Bukhari and Muslim." The questioner is asking: Is there a reason that explains and clarifies why he did not treat her as he (peace be upon him) treated Safiyyah? Why did he not free her and make her freedom her mahr?
I also say: I do not know. But one of the clear reasons is that it demonstrates the permissibility of marrying a slave woman who is not a free woman, and that it is not obligatory for a free man to free a slave woman and marry her. Rather, that is among the virtues of actions. When the Messenger of Allah (may Allah's prayers and peace be upon him) married Safiyyah and made her freedom her mahr, and when he married Maryam the Copt while she remained in slavery, the Messenger of Allah (may Allah's prayers and peace be upon him) combined in these two examples between demonstrating what is better and demonstrating what is permissible. This is what appears to me in answering this tenth question.