The reasons I see the decline in 10th scale is 3 fold.
1. There is no where near the recognition in winning a 10th scale national event as there is winning an 8th scale buggy or truggy race of the same caliber either in the US or World wide.
2. As stated before 10th scale 2wd takes allot more driving skill and if you make a mistake in 10th scale you have a greater chance of breaking it over the 8th scalers.
3. Most tracks are now being built for the bigger faster 8th scales and makes a boring drive with a 10th scale when most end up rolling the majority of the jumps as they are much harder or impossible to do for most.