Question asked why did the prophet salla Allahu alayhi wasalam made zaynab bint jahsh R.A while he married her to zayd ibn haritha he called his adopted son?

 Question asked why did the prophet salla Allahu alayhi wasalam made zaynab bint jahsh R.A while he married her to zayd ibn haritha he called his adopted son? 

Alhamdulillah, Allah says in Quran, 

Al-Ahzab 33:37

And [remember, O Muḥammad], when you said to the one on whom Allāh bestowed favor and you bestowed favor,[1] "Keep your wife and fear Allāh," while you concealed within yourself that which Allāh is to disclose.[2] And you feared the people,[3] while Allāh has more right that you fear Him.[4] So when Zayd had no longer any need for her, We married her to you in order that there not be upon the believers any discomfort [i.e., guilt] concerning the wives of their claimed [i.e., adopted] sons when they no longer have need of them. And ever is the command [i.e., decree] of Allāh accomplished.

So zayd after one year of his marriage with zainab, the marriage ended in divorce because zaynab who was from higher caste saw she is superior to zayd and the zayd even wanted divorce but the prophet told him to be patient until Allah dissolved the marriage and the prophet married her. 

Also this action was to mean one can marry the wives of his adopted son because adoption is not allowed in islam

Allah says, 

Al-Ahzab 33:4

Allāh has not made for a man two hearts in his interior. And He has not made your wives whom you declare unlawful[1] your mothers. And He has not made your claimed [i.e., adopted] sons your [true] sons. That is [merely] your saying by your mouths, but Allāh says the truth, and He guides to the [right] way.

So this marriage was done to confirm the adopted son is not allowed in islam.