I think it really comes down to this:
Dogs, Cats, Cattle...there are vaccines for all of the major illnesses these animals could transmit to one another, and vaccinations are required for them to enter shows.

There are no vaccines for any of the reptile viruses, and many of them are very deadly.

No one wants to risk that. I know I wouldn't. You don't just have animals on display, but in a show, they would be handled by a judge, which would increase the risk of transmitting something (even with hand washing).