Because IndyCar cars would go too fast. This has NOTHING to do with NASCAR cars being "functionally obsolete".
Because IndyCar doesn't have the G-W-C gimmick, because racing back to the caution has never been a part of American Open Wheel Championship racing. Unlike NASCAR, where it was the essence of what American stock car racing is all about.
Without the G-W-C gimmick, drivers have to at least drive with some sanity. That is open wheel. Stock cars need to race back to the caution.
This was awesome:
View: https://www.youtube.com/watch?v=AibV8_0USBA
Listening to the announcer go "umm, umm, umm, it looks like the 7, umm, umm, umm, yes the 7, is the 'lucky dog', and, umm, umm, umm, we will be back for the 6th try at a G-W-C finish, after these messages", isn't.
It doesn't work.
That is the lead excuse that all of the France family's enablers use. Pretty much "it was a fad". Nope. While, yes, there are more choices today, and thus the audience (for anything) is going to be less (unless its spelled NFL) every sport, sans the new sport of MMA, is about where it was relative to all the others, in 1995, or 2005, or 2015. ONE exception. ONE sport has declined relative to all the others. ONE has driven off two of three people who formerly watched it closely. NASCAR. This sport is in trouble.
Mainly because the Frances will not admit their mistakes.