+6 I'm not a diehard NASCAR fan, but I can say from what I have seen that FOX's coverage of any of the NASCAR races is a hell of a lot better than the ABC coverage of the Indy 500. Better HD, better commentators (DW and the guys are hilarious), better interviews.
And let's face it, the drivers have better personalities, or at least personalities more likely to click with the typical American viewer. Douche bags like Dan Wheldon with his popped collar are bound to come off as snooty and drive away most non-Europeans.