Main reason for running conventional forks is stiffness, the usd forks twist too much & go through fork seals on a regular basis on trials bikes.
The first manufacturers to change to conventional forks did so in the early 90s with all of them having swapped by about 95, Beta tried usd forks again in 2000 but they were so bad that a ugly afterthought fork brace came out during the year to stop the twisting, next year was back to conventionals
Current rules allow stationary balancing but not riding backwards, not 100% sure if at world level hopping either wheel sideways while stationary is legal at the moment