I think car counts are dependent on the local situation of the clubs or tracks and their ability to network.
I am lucky enough to live in a hot bed of dirt tracks that are known for producing some of the toughest and quickest racers in almost any division of dirt racing. We have very good "starter" classes such as karting, quarter midgets, and Strictly Stocks (road vehicle turned into race cars) and multiple tracks to support them. We have a variety of support divisions including multiple classes of microsprints (1000cc, 600cc, 270cc, 125cc), hobby stock classes (street stocks, mini stocks, thunder rears), 600cc Modifieds, 305 Sprint cars, and of course Legends. Then we have the "Premier" classes like Big block and 358 Modifieds, Super and 358 Late models, 358/360 Sprint Cars, and of course our "PA Posse" 410 Sprint cars which always are just as fast if not faster than the World of Outlaws when they get here. I can easily count 20 dirt tracks within 2 1/2 hrs of my house varying from 1/8 mile to 1/2 mile, all of which have there own set niches (classes & race night).
With that said, Central PA is bursting with people that want to race. If they can afford it, they will get a car. If they can't keep up with the finances of running every week, they sit until they can race again.
I think the local networking that club members have done in the past few years is what has driven the car counts for Central PA Legends. Since 2012 (my rookie year), we went from averaging 15 cars per race to 22 cars last year. We also have at least 10 new drivers/teams who will be joining us this year. Our willingness to talk to those asking questions and helping new teams get up to speed has not only kept drivers coming back each year, but keeps the new guys from getting frustrated and therefore keeps them from getting a negative opinion of the club/series/ or Legends cars in general. I have heard it many and feel it very often:
"We (Central PA Legends) fight like cats & dogs on track, but are like a family off track."
I believe the drivers/ clubs need to "spread the word" to others to get them interested. Car counts will always be on the downfall unless you willing to show someone why they should race.