What happened? Well, art got more realistic looking for one thing and it started showing exposed breasts in some non-maternal contexts, both secular and religious. That was followed by the printing press which enabled the spread of pornography to the masses. Also, as medical science progressed there were books on anatomy in circulation that were one more blow to the sacredness we held in our minds regarding the maternal breast. Ms. Miles concludes for us, "By 1750 the public meaning of naked breasts was largely medical or erotic."
I have to say, personally, that I have a much easier time (having joyfully nursed my babies for many years) seeing Mary's maternal breast as a sign of God's love for me than crucifixion images. Not to downplay the importance of that sacrifice but it never really brings up those lovey-dovey feelings in me that I am told it should.
The picture above is of Our Lady of La Leche and she has a shrine in St. Augustine, Florida. When I lived down there I knew several people who, when pregnant, made pilgrimage to that chapel. And the date of that devotion? 1600s.
I would love to hear your (respectful) comments on this subject.