I think there are probably several factor for this,some of which were were already stated, but I think one other thing is that alot of collectors now use the internet for searching out items rather than hitting the shows. When a person live 100+ miles from a show, the factor of travel cost, time, etc, all will be considered, as opposed to sitting in front of a computer.
However, nothing can replace the hands-on knowledge you get from attending a show, and the talking to other collectors.