IMHO, the main reason a 5th wheel is more stable is the pivot point between the vehicles is close to the rear axle. Say a side wind, push rear of TV to the side. With the hitch half the wheelbase behind the axle, more leverage to put TV axles out of line with expected direction of travel.
Also with the hitch close to axle, a higher percentage of the weight can be carried by TV without reducing the weight on steering axle. And many can't stand under the idea, lifting the front of the TV can change the alignment enough to make the TV wonder in lane, and corrections can feel like the trailer is in control.