I think a lot of it has to do with the tradition and longevity of the race itself. It is also located in the heartland of America, not to mention it's actually in the heart of the city rather than some farm field in the middle of nothing.
All the great ones have raced there, although it has not been kind to some.
The track looks easy to maneuver around, but it's not. All four corners are different and require a delicate touch. I also think the track blends the American oval style of racing with the European street style of racing so drivers from both styles feel challenged and press to overcome.
The qualifying procedure has been tweeked over the years but remains unique to this particular race
The indy car circuit has been through some tough times over the years and is currently suffering, I believe in large part because of greed and stubbornness, but the Indianapolis 500 has remained a strong constant in that type of racing.