While I can certainly understand collectors being skittish and extremely hesitant to come forward or share their prints, why would they be fundamentally against scanning them?
Some are extremely cautious of the legalities of copying a print. In the past studios have been known to confiscate privately “owned” theatrical prints, as they technically remain the property of the studios permanently. Depending where you are the law might actually consider it stolen property, and you can be charged just for having it in your possession. Also, studios like Disney and Lucasfilm are known to aggressively pursue their IP. A friend of mine made a Lego Hoth display (without a kit) in the 90’s around the time of the Special Editions, and a local shop displayed it purely as a piece of fan art. Both he and the shop owner received C&D’s and threats of litigation from Lucasfilm.