Where do you race Jim, as I know of several tracks locally that ran nitro onroad and had multiple classes.. 1/8th, 235, 200TC and sometimes open and spec 3 port motor versions as for offroad there is buggy, truggy, gas truck, muggy and monster truck aka tmaxx now at big races I have seen sportsmen intermediate and pro versions of monster truck, buggy and truggy like the psycho nitro blast with 350 entries. or the RC pro series which I think has sportsmen open and pro.
I think you see big 8th scale races like the Roar buggy nats have such turnout is because of the cost related to travel is not a prohibiting factor to your average 8th driver. I have a cargo trailer to go to 8th scale offroad races. When you spend $400 on a good motor or 2 and $500 or $600 on a good car plus parts cost more on average. $150 each on servos $100 on a good pipe $25+ a gallon on fuel M3 tires cost $25 or $30 a set. What's $2000 more to get a trailer. More likely 8th scale drivers due to the inherent costs of the class alone have the means to travel. Electric can be had for $300 tc5 new $200 motor speedo $50 servo $30 spec tyres last longer over all $80 lipo now a days about half the price as 8th when you add it up. More conducive to local club racing and not traveling.
Granted I'm an exception to the rule as I have traveled 500 miles to run a club race for onroad and traveled weekly 200 miles oneway to run a points series in another town for 12th scale.