But my sense of the Indy500's decline had to do with split of the old Indy car racing league (CART) splitting up into two leagues. The league with the stars couldn't race in the league which had the rights to run the Indy 500.

Indycar and to a lesser degree F1 racing's popularity has declined in the last 10 or 15 years. Neither sport has American born stars outside of Danica who has finished well at Indy, but never won.