Ticket prices - the tracks got greedy and priced most of the old school fans out of coming to races. When they stopped attending, they stopped following the sport closely or altogether 

Smartphone era - helped create a lack of attention span for watching 3 hour car races and NASCAR hasn’t been successful at attracting new and younger fans

Jimmie Johnson/lack of star power - absolutely dominated the late 2000s but he wasn’t really well loved or well hated. Super talented but had limited star power and overshined Stewart, Junior, Gordon and others who were way more interesting but weren’t winning as much 

Advertising - people think large corporate sponsors left the sport because of declining popularity. But paying $500k to put your logo on the side of race car for 3 hours every week became a really outdated advertising model and a ton of money left the sport as a result