The issue of the popularity of the Roman Catholic view of Mary came up in another thread. There was widespread opposition to the Catholic view of Mary in early post-apostolic church history. The Catholic view of her, in its entirety, isn't found in any extant document of the earliest centuries, despite agreement with some aspects of the Catholic view among some sources. I have several articles on early Christian views of Mary here. We've discussed the mother of God issue in the comments section of the thread here.
Concerning the Protestant reformers, it's true that some of the early Protestants, such as Martin Luther and Ulrich Zwingli, held a much higher view of Mary than most Protestants do today. But the extent and significance of that early Protestant agreement with the Roman Catholic view of Mary is often misrepresented.
The Catholic Marian scholar Michael O'Carroll notes that Martin Luther was "not wholly consistent" in his beliefs about Mary (Theotokos [Wilmington, Delaware: Michael Glazier, Inc., 1988], p. 227). O'Carroll writes that Luther "underwent a certain development in his ideas, and we must not forget that, up to his middle thirties, he had accepted - though with some questioning - traditional Catholic ideas and practice in this area" (p. 227). Though Luther was "emphatic on the divine motherhood" and "true to Catholic tradition on the virginity" (p. 227), for example, he also "talked of the danger of making Mary into an idol, even a 'goddess.' The 'papists' have done so....Of the feast of the Assumption, he had said: 'The feast of the Assumption is totally papist, full of idolatry, and without foundation in the Scriptures.' He even said that he would keep the Visitation to 'remind us that the [Papists] taught us apostasy.' The Salve Regina, Europe's most powerful Marian hymn, he dismissed. It said too much." (p. 228) However, O'Carroll gives examples of other comments Luther made that were more positive toward Roman Catholic Marian beliefs, sometimes in a seemingly inconsistent manner.
O'Carroll says much the same about Ulrich Zwingli (p. 378). Like Luther, he seems to have accepted most of the view of Mary that was popular in his day, and he seems to have been inconsistent. Despite some positive comments about popular Roman Catholic Marian belief, Zwingli also "was against all invocation of Mary. He denied, on the Reformation principle of sola gracia, all merit on Mary's part and any power of mediation or intercession on our behalf. He waged war on all images." (p. 378)
O'Carroll describes John Calvin's view as much closer to that of modern Protestants. Calvin condemns the "gross and abominable superstitions" in the Roman Catholic view of Mary (p. 94). He comments that "their insane raving went so far that they just about stripped Christ and adorned her with the spoils" (p. 94). Calvin wrote, "It is they who do Mary a cruel injury when they snatch from God what belongs to him, that they may deform her with false praise." (p. 94) He criticizes Marian relics, and he "held that Mary was the Mother of God...Yet he scarcely used the title Mother of God and, in a letter to the Calvinist community of London, he discouraged its use. 'To speak of the Mother of God instead of the Virgin Mary can only serve to harden the ignorant in their superstition.'" (p. 94) Calvin "rejects totally the Immaculate Conception (qv) as he does the Assumption (qv). He thought that the latter feast had one advantage - Catholics thinking that Mary had been assumed bodily could not worship her relics....Invocation of Mary he forbids....He brands all invocation of the Virgin execrable blasphemy. He attacks, too, holy images of any kind, therefore of Our Lady, calling them idols" (pp. 94-95). Calvin believed that "Mary will take her revenge, on the last day, on those of whom she is the 'mortal enemy', the Papists" (p. 94).