The reason we don't call Mary the mother of God is because the Holy Spirit didn't call her mother of God.
Demonstrably untrue so there must be another reason.
If you have even a rudimentary ability to read and understand Hebrew for yourself bring up the highlighted in bold portion of Isaiah below and you can convince yourself, if both you and God allow, that Miriam is indeed the almah/parthenos/virgin who bore a son (not Israel, except through Miriam) and his name was called ... (notice the two names) Jesus (in Matthew, English, not original Greek) and immanu el (in original Hebrew, which is actually, pay attention here, two words in Hebrew text). One cannot, in truth, call Mary the "mother of Jesus" without also allowing "mother of Immanuel." What does Immanuel mean ? It seems to me you will confess that the Holy Spirit calls/interprets this name as literally, and in truth, "God with us." It seems to me you use English commonly, so Miriam is the mother of God with us, and she is the mother of Jesus, and those names are in unity. Moreover the Lord spake again unto Ahaz, saying, Ask thee a sign of the Lord thy God; ask it either in the depth, or in the height above. But Ahaz said, I will not ask, neither will I tempt the Lord. Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost. Then Joseph her husband, being a just man, and not willing to make her a public example, was minded to put her away privily. But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. And she shall bring forth a son, and thou shalt call his name Jesus: for he shall save his people from their sins. Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
יג וַיֹּאמֶר, שִׁמְעוּ-נָא בֵּית דָּוִד: הַמְעַט מִכֶּם הַלְאוֹת אֲנָשִׁים, כִּי תַלְאוּ גַּם אֶת-אֱלֹהָי.
יד לָכֵן יִתֵּן אֲדֹנָי הוּא, לָכֶם--אוֹת: הִנֵּה הָעַלְמָה, הָרָה וְיֹלֶדֶת בֵּן, וְקָרָאת שְׁמוֹ, עִמָּנוּ אֵל.
And he said, Hear ye now, O house of David; Is it a small thing for you to weary men, but will ye weary my God also? Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel. Butter and honey shall he eat, that he may know to refuse the evil, and choose the good. For before the child shall know to refuse the evil, and choose the good, the land that thou abhorrest shall be forsaken of both her kings.
-Isaiah catholic chapter seven, Protestant verses ten to sixteen as authorized by King James, Hebrew from http://www.mechon-mamre.org
-Matthew, catholic chapter one, Protestant verses eighteen to twenty three, as authorized by King James