[skyking]

If torsion axles are so great, why do the serious off roader's pulling trailers choose leaf springs?

It relates to tandem axles, and how a torsion axle pair does not equalize over rough terrain and approach angles.
A well -lubricated set of equalizing spring hangars does a great job of trying to keep an equal weight on both tires.
Torsion axles do nothing at all. In fact, you can put an 8" tapered block in front of either axle, drive up on it, and the other tire will come off the ground and you can change it without a jack!
So in a truly lumpy environment, the conventional springs will keep a better even pressure on the ground in trying offroad conditions.
Single axle, the problem is moot.

Tandem torsions rule the pavement, if set up flat.