Why do Protestants not refer to Mary by the title "Mother of God"?

I have heard the title "Mother of God" in connection with the "Hail Mary" prayer that(Ave Maria) is recited by Catholics. However, I have never heard this term used in any Protestant setting. (From the comment by Bobo, we find that the Orthodox also refer to Mary in this way; *Theotokos* in Greek literally means "Birth-giver of God", as well as *Bohoroditza* in Russian. Both of these terms are widely used in their respective Orthodox groups). Why, then, do Protestants not use this title that appears to be so common in Catholicism?