I think it really comes down to the fact that these single wheelers don't support the operator without power. You can stand on skateboard, slow down, and come to a complete halt and it's still working as intended. On a bike you will eventually topple from one side or the other, but the way a bikes works means the rider is supported in a natural position for a long time.

When the one-wheeler loses power it just nosedives, the front hits the ground and they halt immediately.

I wonder if they could save some of the danger by placing small, unpowered wheels in the front and back so when they dip they can at least keep moving.