Adoption and Zainab
In order to marry Zainab, Muhammad abolished adoption and all the goodness that came from it. Adopted children were now seen as strangers living in the home with no rights to take the family name or inherit as a regular child would. As well, the rules of marahim (blood relation) made it difficult to be alone with this child. The end result is that Muslims hardly ever adopt kids. Allah wanted to ensure that everyone knows that they can marry their adopted-son’s ex-wife so he made Muhammad marry Zainab (or maybe Muhammad liked her).