Because Mary is so associated with the Holy Catholic Church, and the only thing Protestants agree on is that they think the church is wrong.
C'mon! You don't really believe that, do you? There are a great many things we agree on.
Speaking as a former Catholic, now an Evangelical Protestant, can you please explain to me: Why would I reject the Catholic dogma about Mary's sinlessness if it were truly a divinely-revealed truth? We hold probably 90% of the same beliefs as Catholics and the Orthodox do. Why is it some Catholics insist it is because of a hatred of Mary and the Catholic church that "Protestants" disagree with them about the IC? I deeply admire, respect and love Mary and her courage and faith are a wonderful example for us all. I'm looking forward to getting to know her in Heaven.
Gee, Tex.
With a brush this broad the fence will be painted in no time!