Money is a big issue when it comes to booking guests, and I'd be willing to bet that an established convention's attendance numbers in past play a role in forecasting how much to commit where. The last thing you want to do is book a lot of high-dollar guests and then come up short of funds to pay them. From what I can recall, BABSCon scaled back its guest list pretty heavily this year in order to avoid operating at a loss.
There's also the question of availability. VA's and their agents often work out schedules months in advance--con appearances, recording sessions, personal time, what have you--and setting something up on short notice is a dicey prospect. I think I read somewhere that both Ashleigh Ball and Andrea Libman had other commitments for the weekend of BronyCon, but don't quote me.
At the moment, this is shaping up to be only the second con I've gone to (out of 10) that doesn't include at least one of the Mane Six actresses. Unusual, yes, but them's the breaks. I'm not going to lose any sleep over it, though; they were all there last year, and three of them turned up at BABSCon this past April.
As for my own personal wish list: I'd like to see Britt McKillip and Andrew Francis, and also G.M. Berrow. I still have one more Daring Do book to get signed, and the younger of my two nieces thinks I'm the greatest thing on two legs because of all the signed chapter books I keep bringing her.
(Edited to plug a small hole in my memory.)