I think its most of the things that people have said.
Cost - add up a tank of fuel to get there, one to get back, a hotel room, a bar bill, food and you're hitting £300 for a night and a day out. Put that into the grand scheme of things is a fair chunk of cash that most people think twice about spending on a MINI run.
Got the T-shirt - been there, done that! Have done most of the big ones now, and I know which are my favourites and which I'd rather spend the money on if I am going to do a MINI event. But if its the same place over and over you tend to think well thats the one I'll not do this year, if the venue isn't anything that special then you think I'd rather have a weekend away somewhere nice for the same cash. I do appreciate that these runs take a lot of organising, and they have shown me some of the most amazing places such as Ireland, Scotland, Yorkshire Dales, Northumberland, most of the country infact. The social is now just as important to me.
Then there are all the other things, the MINI isn't as special as it was, its very main stream, people move on but the events don't seem to have done that. Its just the same old same old, a few new people come along, it becomes too much of an effort, there are more idiot drivers who are more worried about how good they look on video and people think why take the risk of having idiots like this cost me my car, things like that.
I don't think runs will ever die, as look at the classic mini, they still do it, but they do it the way they have learnt to do it, and make it last. But I think that as there are increasing fuel prices, younger owners with smaller budgets runs need to change, camping like on South Downs, making a weekend of it, making it a social more than a race around country lanes on mass. In fact South Downs for me was fantastic as it covered all those things.