Okie, that's similar as to why baseball cards from the 50's & 60's are worth much more than cards from the 70's onward. Aside from the cards being flipped against walls and general damage like getting clothes-pinned to the spokes of their bikes, etc. A lot of kids back then kept their cards but when dear ole mom got tired of seeing the kids room in a mess, into the trash they went.
Also remember that nobody cared about the posters back then, let alone keep them safe and secure. Of course some theatre managers with an eye for attractive art did keep them. The posters that remained at the exchanges were sent out, returned (usually), and resent again and again until they were falling apart. So many variables in how these posters have disappeared over the years.
And I would love to see that Dracula reissue 3-sheet in color.