Posters, since 1940, have not been thrown away. Comic books by contrast were tossed. Moreover, children had a close emotional connection with comic books but none with music or posters. The hottest collectibles of recent years have been comic books, baseball cards and toys, and childrens' items that kids loved. There are both psychological and historical factors connected to this.
Children revere what their parents dislike and throw away.
Similarly, there has been a huge gap between what cultural arbiters have said is good and what people have actually liked. Posters remain in the category of collectibles so new that demand is everything in so far as price. This is not true with stamps, coins, and other mature hobbies, where scarcity creates a demand for items. With new collectibles, emotional demand is everything. The trick then, perhaps, is to cause posters to gain prestige regardless of how someone feels about its beauty, star, etc.