Blair is 100% correct - and the problem is even deeper than the sportsman classes. NHRA seems to refuse to see whats right in front of their eyes - the sport itself has shrunken dramatically over the years. There are so many factors that its hard to put together a coherent story of "why", but I would wager that drag racing in total is probably 50% of what it was in the 70's & 80's.
The growth of big time sponsorship and giant publicity for the Pro classes in the 90's and 00's masked the underlying decline in participation and interest in all other levels.
The well intentioned "Super" classes were (and are) near impossible for fans to understand or relate to, and they dominated the hours spent at a national event, leading spectators to feel disconnected - why did they slow down, why are they sputtering, why did the slower car win?
The cars - even at the Stocker level - have become so professionalized and so highly developed that the average spectator is intimidated away from even trying to step into the game. To the uninitiated it looks like you need a toterhome, a 38 foot stacker, and four days per week off in order to run at the entry level.
Pro Stock started out as an extension of Super Stock or Modified Production style cars, but from the stands they look more like really fast Super Comp cars now, with unidentifiable bodies and exactly perfect identical runs from all involved. There is nothing visceral about watching them, and the incredible amount of technology is largely hidden from the casual fan. The cartoon like Pro Mod stuff with various drivelines (turbo vs nitrous vs blower) are far more entertaining.
Personally I think they should push the current Pro Stock cars into the Pro Mod pool with some weight shuffling to bring everybody into parity, and have a N/A option for the big tire bar of soap type cars, and take the gloves off of the "Factory Stock/new car Super Stock" cars like the new Camaros, Challengers, and Mustangs & make them the new Pro Stock. It would be closer to the roots of a class which has long since lost its way.