IMO, four things have greatly contributed to NASCAR's lower fan interest...
1. The Car of Tomorrow. Stock cars are no-longer STOCK cars and fans don't personally identify with them. The old adage "win on Sunday, sell on Monday" no-longer applies.
2. Too many Saturday night races and race starting times being all over the map.
3. The selling of TV rights to FOX and NBC who have put most, but not all, of the races on FS1 and NBCSN.
Between #2 and #3 above, even die hard NASCAR fans are having a hard time actually finding the races on TV. Cable cutters (many blue-collar fans) have been left out.
4. The excessively high ticket prices (and other charges like parking) to attend the races in-person, the requirement to purchase season tickets to get the better seats, and (especially) the introduction of PSL seats at all of the tracks (a kick in the head to NASCAR's largely blue-collar fans).
I disagree that the new format is the cause. I actually like the 3 races within a race format, which is kind of like the heat races in local stock car racing. It makes the drivers more competitive throughout the entire race and brings the team/pit strategy more into the race. I also like the playoff/elimination format, it keeps one driver/team from running away with the championship when the season is still far from over.