It goes without saying that Protestants and Catholics have a very different view of Mary. Protestants view Mary as a devout woman chosen by God to bear His Son, but still a sinner just like all the rest of us. Since the Immaculate Conception is not expressly found in scripture, they reject it. Moreover, in the view of many Protestants, the Immaculate Conception would not have been necessary because Jesus did not have Mary’s DNA. She was merely an incubator for flesh that was exclusively of the Holy Spirit, not a biological mother. There is even one school of thought that maintains Mary could not be the biological mother of Jesus because if she had been, Jesus would have inherited original sin. Imagine that!
Protestants reject the Perpetual Virginity of Mary, even though it was affirmed by no less than Martin Luther, John Calvin and John Wesley. They truly believe that Mary was the mother of at least six other children, maybe more.
Protestants regard the title Mother of God as blasphemous because they can’t (won’t) wrap their minds around this concept that actually transcends time.
Finally, Protestants reject the idea of the Assumption as being outside the Bible. They also reject the idea of praying to Mary or any of the other saints.
So, they believe Mary was a good woman given to a unique role, but one who lived and died and will be resurrected along with the rest of us at the end of the age.
Now, compare this with the Catholic view of Mary who sits at the right hand of Christ as Queen of Heaven, and who makes constant intercessions for us, and you see the two groups are talking about two different people. Protestants love and respect what they regard as the Mary of the Bible, but they do have considerable hostility toward the Catholic Mary who they believe is either a goddess of something so close to being a goddess that honoring her would be sinful. Sort of like honoring the pagan Minerva or Diana. Those Protestants who hate Mary regard the Catholic Mary as equally pagan.
Of course many Protestants just ignore Mary and never think about her except at Christmas.
This is pretty much what I was taught growing up as an evangelical Protestant.
Although Lutherans (Luther), Reformed and Presbyterians (Calvin, Knox), and Methodists (Wesley) know about the Early Protestant Church founders (although they don’t necessarily know what they taught), most evangelical Protestants, including many of the non-denoms, know very little or nothing about these men and their teachings. They have no idea that they believed in the perpetual virginity of Mary and that they accepted her as the Mother of God per the Church Council.
Even if evangelical Protestants do know about the teachings of the men mentioned, they would reject those teachings if they were in conflict with
sola Scriptura. And since the perpetual virginity of Mary is not clearly spelled out in the Bible, and indeed, seems to be negated by the mention of Jesus’ “brothers and sisters,” evangelical Protestants would reject teachings about Mary from Luther, Calvin, and Wesley.
In answer to the OP, one of the reasons why I believe there is hostility to Mary is that she is a champion of the unborn children. Mary was the epitome of the “crisis pregnancy,” and had she lived today, would have been counselled by many to get an abortion. Mary is very threatening to those who advocate “choice” for abortion. She is proof that a child that is aborted may have turned out to be someone very important in history.
Finally, I believe that many Christians are hostile toward Mary for spiritual reasons–she is the one who leads people to her Son. I think that non-Catholic Christians sense this in their spirits, and resist the pull to the real Jesus. They would rather continue to worship the Jesus that they have made up in the imaginations or in happy Gospel songs. They want to ignore the suffering Jesus on the cross–remember, Mary stood by Him during this time. But many evangelical Protestants will state vehemently that Jesus is no longer on the Cross and that a crucifix is not necessary. You see, it’s nicer and more cheery to believe in a handsome, smiling Jesus rather than a suffering, bleeding, dying Jesus. Mary leads people to the REAL Jesus–ALL of Jesus, including the Eucharistic Jesus, and this is threatening to those who want to continue to practice “personal Christianity” and decide for themselves what the Bible teaches.