Is there a Shari'ah reason or explanation for the Prophet (peace and blessings be upon him) not freeing Maryam al-Qibtiyyah?

The Shaykh : As for the second part of the question, which is his saying, "Is there a Shari'ah reason for the Messenger (peace and blessings be upon him) not freeing her and marrying her?" It is as if the questioner is asking why the Messenger of Allah (peace and blessings be upon him) did not treat Maryam al-Qibtiyyah 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), and this is in Sahih al-Bukhari and Sahih Muslim. The questioner is asking if there is a reason that clarifies and explains why he (peace be upon him) did not treat her as he treated Safiyyah, why he did not free her and make her freedom her mahr. I say, I do not know, but one of the clear reasons is that in this is an indication of the permissibility of marrying a female slave who is not free, and that it is not obligatory for a free man to free a female slave and marry her; rather, that is among the virtues of actions. When the Messenger (may Allah’s prayers and peace be upon him) married Safiyyah and made her freedom her mahr, and when he married Maryam al-Qibtiyyah while she remained in her state of slavery, the Messenger (may Allah’s prayers and peace be upon him) combined in these two examples between indicating what is better and indicating what is permissible. This is what appears to me in answering this tenth question.