After much thought, I'm convinced that the lightness of good tubular wheel/tyres is the main reason they ride well on rough pavement.
Lower the bike's mass, and it will better stay on the ground surface while a heavier bike achieves sufficient upward momentum to leave the ground against the rider's weight. I have a bumpy downhill on my way out of my neighborhood and I immediately feel a heavier bike pounding the ground uncomfortably under foot, hands and saddle.
David Snyder
Auburn, CA usa