Remember the context of the time: Luther and Calvin were both Catholic priests before they reacted against Rome. Those things that were relevant to worship, they kept. Those things that they deemed hinderances, they tossed.
In researching the development of Marian dogma, I've found that most of the first Protestants held a very high opinion of Mary. Both Luther and Calvin believed in her perpetual virginity and divine maternity, Luther believed in her immaculate conception, and Oecolampadius even taught that she was the Mediatrix of all graces! Why has this switched around to the point where I've heard many Protestants discourage even talking about Mary?
That same tradition continued post-Luther, post-Calvin. Even more than the Reformers, however, were the Radicals (sometimes known as the Anabaptists) and their maniacal fervour to reduce Christianity to some basic sediments, and dispense with the froth and foam. They considered Luther heroic, yes; but they also thought he didn't go far enough. Hence they set in motion a type of puritanism that acted as a distilate to anything beyond the pale of scripture, preaching, and symbolic sacraments. Thus Mary, while respectable, really was only instrumental insofar as she birthed Jesus. After that, she's little more than a biblical after-thought.
Carry that same creeping puritanism forward to the present day, and you have some memetic tendencies in Protestant circles to dispense with Mary altogether because she seems to get in the way of Jesus by being part of a grammar people are afraid will lead to Catholicism.
Despite my hearty agreeance that Catholicism is a frightening thing, for most Prostestants it is an evil thing. And if Mary is going to have the dogmatic fortitude to be mediating between Jesus and the rest of the world, then the misgivings of Protestants will no doubt exculpate her from such a nepotistic scheme, and set her where she belongs: in a manger, and at the foot of the cross, and no more.