Torsion suspension is usually found on lighter weight trailers, Airstream being the exception. They’re also usually lower to the ground. Leaf springs are actually an antiquated design, cheap to produce in numbers, and can be used on heavier rigs as they have been for years. They do make torsion suspension for heavy rigs, but I don’t know of any RV’s that use them.
Our first TT was a Jayco with torsion suspension. It actually towed very well. To use torsion suspension on larger heavier trailers, Lippert and Dexter would have to redesign the frame mounts which they’re not going to do as it costs money and they can’t justify the expense as leaf springs do the job now.