I think it's the cost of a competitive engine nowadays thats killing the touring car class. People who were willing to fork over the cash in the beginning are starting to see the light now, and new blood aren't even considering it because they(or their parents) can't afford it or arent willing to fund it.
Lets see.....spend close to $300 for a .12 on-road engine for my touring car just to stay competitive, or spend $120 or less for an competitive off-road engine for my stadium truck for just as much fun, or even spend only $400+ for a fast and fun COMPLETE RTR monster truck. Serpent's lower cost Mega engines are a step in the right direction but it's still hard when the pricey Novarossi based engines are the most sought after.
IMHO, I think foam tires(and the required equipment & knowledge) had a hand at it also.
Remember Nitro TC's popularity back in the beginning with the rubber tires and $100 engines? It's growth was phenominal!
I love on-road racing more than I do off-road, but if the cost trend continues in Nitro TC, i'll be coming home dusty and dirty on sundays a lot more often.
BTW, The only other thing I can think of is the higher heat during summer(heat reflected/radiated off the asphalt, etc..) are turning people off. It sure gets HOT here in Hawaii even under a tent in the middle of a parking lot.