Wow what a provocative post, enough probably to start another thread.
My opinion follows (background I am a longtime fan of all motor racing starting with Indy in my formative years and expanded to F1 and endurance with occasional interest in NASCAR).
Why has general US interest in Indy, NASCAR and F1 declining? Simple, because the racing is BORING (to the average American). Yes there is a lot of interesting things going on in the backstories but only the diehards follow that. The actual races themselves make for awful television and that is what counts in the US.
Technology equalizing all the cars, homogenization and boring drivers is not a good formula. Back in the golden age of Indy almost every team had a different chassis, PARNELLI , Eagle, McLaren, Coyote , Wildcat , PENSKE , LOLA, Reynard, MARCH ,CHAPPARAL. We had F1 teams with Lotus and McLaren and Brabham, multiple chassis, Offys. 4 Cam , Foyts, Turbo Chevies, Stock Block Chevies, Turbo Cosworths, and even 4 Cam V8 Drakes, all racing against each other at the same time. Man those were the days. Now you have spec cars and the powers that be decided that the focus should be on the drivers not the cars. Problem is the drivers are boring.
NASCAR is suffering the same fate. Spec cars and they are all equally as fast with speeds and passing limited by aero push. The only racing occurs in the last lap. Life is too short to watch a racing like that.
Rant off and I am probably way off base but that is why that is called an 'opinion'.
--jd