There's Shear forces between the parts of a threaded headset's 2 nuts.
pulling against each other .
OTOH, with a threadless headset the forces keeping the top race adjusted
is compression .

the top nut is inside the steerer tube and the bolt thru a top cap
is forcing down on the stem and the stack of spacers underneath it.

the adjustment is held by tightening the bolts holding the stem to the steerer tube.