I've never been there but every year since it's possible, I follow the livestream!
I think the historical background does a lot in the success, the first edition of the show was in 1898!
Aachen is a part of the four shows of the Rolex Grand Slam of showjumping with Geneva, Calgary and 'S Hertogenbosch, all of them being prestigious shows with amazing venues!
Every top rider has been there, and it's a dream for all of them to win the CHIO's Grand Prix. The date is also very well chosen, just before the World Equestrian Games! It's a great rehearsal for riders, and a way for national trainers to support their choice regarding the riders they selected for the Games.
The stadium can host 40 000 people, and there were more than 100 000 people last year during the 3 days of the show, and the ambiance seemed to be so cool!
There's also a true lifestyle around the event, with a village that can attract non-riders as well.