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barrysloateA possible answer, but not necessarily the whole answer: in the case of vintage posters, because they are so large and brittle, it is a near certainty that they will need some kind of restoration, so it has become an accepted practice. With cards, they have generally survived over the years with nothing more than natural wear, so collectors expect them to remain as is. I know there is more to it, and certainly the advent of the slab has made card restoration taboo.
Another aspect of restoration, say with posters and large format photographs, is that it is important for their long term survival. Deacidifying brittle paper, using Japanese tissue to repair tears, and other practices will give posters a longer life; likewise, cleaning the oils and chemicals that are embedded in an albumen photograph will likewise prevent it from decaying over time.
All of this still begs the question of why cards can't be restored, but it is a start.