Ah just remembered, there was no room for a clamp directly on the steerer a la aheadset. The only clamp was the one one the riser, and that tends to loosen like you also found. I had the identical problem on my Raleigh 20 years ago. I concluded the riser will necessarily loosen due to the flexing - basically it shoulders itself away from the headset bearing, and no amount of clamping will solve that. At least that's what I concluded. So the longer steerer gives the necessary space for a clamp just on the steerer to hold it in place, independent of the riser's clamp.