Because the cars aren't something you can drive on the street, and the drivers aren't like people's cousins. IndyCars don't relate to the average American.
OT, I know RHR is the only American winning races lately, I wish he'd rep California instead of America.
None of this really mattered 20 years ago though. 3 of te first 5 Indy 500 winners were foreign. Granted, 21 to 65 the race was won by an American as well as 67 to 88. From 89 to now, an American has won just 5-6 times. The race and series were just as popular during 89 to 95 span and previously to that.
The issue now seems to be the godawful TV contract. Or at least the main issue. Nobody knows where or when to find the race because they don't race every Sunday for 30 something weeks. And when they do race on Sunday it's up against NASCAR. Any momentum built up is lost from week to week when they're off.
My girlfriend went to her first IndyCar race at the 500. She was hooked after watching Brazil. The thing she points out all the time is how much better looking all the drivers are. Her word is gorgeous. I think they need to use that to their advantage to reach the female fan base.
The movie Turbo is going to be fantastic advertising for the series. They have an incredible opportunity to reach kids. Hopefully they can capitalize on tat too.
Ultimately, racing as a whole needs to reduce the cost of going. Slash tickets prices, offer cheap packages, do whatever it takes. The money needs to come out of the track owners and promoters pocket and back into the fans.