For me its cost. Most shows are a good 180 mile round trip, witch the mini can easily do. So that's a tank of fuel, plus entry for 2 people comes to another 20 odd quid. And for what? Half the amount of traders (some of witch have disappeared for whatever reason over the last few years) half the amount of club stands and cars.
Or to put it in perspective of how expensive mini shows have become. I went to a country fair/steam rally/horse show and classic car show locally (20 miles away) that attracts over 40.000 visitors over 2 days with over 1,500 exhibitors and traders.
It cost £20 for 2 adults (under 16's are half price or under 5's free) to get in and you had over 50 Steam engines of all sizes, tractors, classic cars (with the odd mini) and bikes, buses, military vehicles, heavy horses, funfair, live music, spitfire and hurricane fly past, car boot/traders (I couldn't pass us packs of imperial nuts/bolts for a £1 a bag, cheap tools and consumables and even found some NOS mini spares for pennies) Plus loads more going on. And even the food was reasonably priced, even if the queues were a little long.