Do you mean what attracts exhibitors, or what attracts audiences?
I've shown in one county fair, and would have shown in others except they're being cancelled right and left around here...very sad. I think Mingus shows well against the big horses (our fair only had open classes), and I like the relaxed atmosphere, so we'll go back next year.
To market your show to exhibitors, especially to youth and amateurs, I would first organize a miniature horse club in your area if one doesn't already exist. Before the show, I would hold a clinic to help those who are new and possibly afraid of showing. Give them a chance to practice grooming and clipping, getting their horse out around other horses, showing them to others...give them loads of feedback in a no pressure atmosphere.
Again, for kids, how about coordinating a transportation board to help those who don't have it?
Money talks, but I don't think it's why most people show. I personally would rather see that money put toward creating a better show, perhaps bringing in better judges.
As to what attracts audiences: good publicity...and it doesn't have to cost very much at all
We just returned from the Oregon State Fair, where they have finally gotten a clue that people, especially families with children, LOVE minis. This year there were signs all over with arrows saying "MINIATURE HORSES." It worked. The classes were well-attended and there were many visitors to the barns.
They could do even more by running a photo and short article about minis in the newspaper insert about the fair.
The audience at the Oregon State Fair LOVED liberty, so I would highly encourage that and other fun classes
Minis are a great way to draw families to the fair, bringing in more money, which most fairs are sorely lacking. Fair directors are caught between the past and the present and don't know how to effectively market to their target audience.
Good luck!