Why?
Because it (IMO) better describes what the council was trying to say. Mary being the Mother of God has little to do with Mary, the whole focus was on who Jesus was. Jesus is True God and True Man, and Mary was His mother. So, since Jesus is one person, and Mary is His mother, you can call her Mother (or Bearer) of God. It never meant that Mary was the source of the second person of the Trinity, but that Jesus really was true man and true God from the moment of conception.