I think it is because external horns are, 1) Very easy to install and rig, 2) Very very effective, with minimum play/slop. Both important qualities in todays high performance ARF jets.

Torque tubes on the other hand have the advantage of being 1) Internal, (low drag). 2) Invisible. The down side is they are much harder to install, and, if not done corretly can induce play/slop.

However, if torque tubes are designed and installed well they are just as effective as direct linkage. All the BVM Bandits (except the BARF) use internal torque tubes on elevator, and these are some of the fastest jets around.

And for scale fidelity, torque tubes are the only option.

Roger