Much as I hate to say it, but sometimes they just don't...
There could be any number of details, any one of which on another day might not matter at all. For instance, it's quite a way North, and an early ish start. So for all but those on the doorstep, it means leaving from home 7am or earlier. It's late in the (blatting) season, people tend to leave it later before committing so that they can see what the weather will be like. It's smack in the school holidays season, so many are either away, or just returned/just about to go.
The info we give to AO's in similar situations, are the key things are to just keep plugging away with runs out; not everyone will make every run, but do enough, and sooner or later, everyone that's interested will get a chance.
And if trying to make a bigger, "event" type run, it takes time and a lot of work. It takes a lot of advance planning and more importantly plugging and pushing of the event. Getting it in to calendars early enough for people to plan around, then actually building a head of enthusiasm to get them to sign up. Even so, the first event, or even the first couple (if it becomes annual/regular) may just have modest numbers. But, if that first one goes down really well and is well employed in "marketing" the next one, numbers will grow.
Spur of the moment events rely on having a pool of local members who are available and enjoy such last minute things, and many areas don't have so many members in that boat. (Young families, younger members etc all add commitments).
But to go back to your route; it's one I'd love to have a go at, but for me, it would be a three hour drive to even get to the start! And I'm not that far away. I'm sure there are others that would fancy it that are in a similar position. (Not suggesting you should change anything, btw, just what affects people's choices).