Last year's footing, I have a feeling has a LOT to do with this. The footing at Chatt. Hills was ROCK HARD CLAY and many did not run xc due to this last year. In addition, the Advanced championship was not a championship course. It shared many fences with the Intermediate. Five Points and Plantation are both in Sept. and have better footing. So many horses ran Richland as well. The AEC being in GA is just so far out of the way for many. With so many going overseas this fall, I doubt driving to GA is in the cards.
The UL divisions have always been thin at the AECs, and rarely filled with "championship" caliber horses, despite various ULRs making special efforts to bring their horses and students and support the event in different years. Maybe it's the timing in relation to fall three-day schedules, maybe the location is sometimes a problem, but it seems as if it's never really taken hold at I or A.
All of the above, plus I wonder if it just isn't that financially efficient for them? At an average local HT they may be riding multiple sales horses and clients' horses, but at AECs they may just have the qualified horse to ride. They could be leaving money on the table by leaving home for a long-distance event with just 1 or 2 horses or clients.