I suppose it must depend entirely on the people who organise the show. There are categories at our local show for Red, Rose, White and Fortified wines, and as far as I know, no special licencing is needed: they are shown, not sold.
In your case it could be any reason, from "we can't find someone expert enough to judge it" right through to "We're all Wee Free here and think alcohol is an abomination"!