Not everyone wants to, or should go run a KISS or USAC show. And too many cars can hurt a show just as much as too few cars. Especially if many of those teams don't have the experience or funds to be competitive with the top teams.
You all have no clue how lucky your are in Indiana to not have a million classes to dilute the fields with the majority of tracks have 5 or 6 eight car features and be able to have a track run a big special and other tracks still have enough cars for a show that night for the racers and fans who can't make the special race.
And don't forget something else. When all the well funded teams go run the KISS or USAC shows the other tracks give the less well funded and experienced teams a chance to shine and have their night in the limelight. And what's so wrong with them having a chance to run for a little more money the night the top teams are running elsewhere? Sometimes that little bit of extra money, and exposure to fans and sponsors can make a huge difference to a small team.