It's all about the big picture. I don't think you can isolate it to one, or even a few things. You could probably list 20 things that made "golden era" Gibson guitars unique and near impossible to 100% clone. That being said, most of the wood for the Les Paul guitars was a unique dimension and was not purchased from the same supplier as most of the product line. I believe the maple was also sourced out of Ohio, not Michigan. As a result, I do not believe the maple, and maybe even the mahogany, was air dried like some of the other wood. There is also the argument that a few of the Les Pauls (Particularly early ones) had other wood, though similar in appearance, that was not mahogany.