Could it not be that, relative to the mass of the aircraft, a 707 has smaller, and more, energy-absorbing protuberances to be ripped off in a ditching than a twin-jet?
Just look at the head-on, gear-down, profile of a 707 or DC-8 and compare it with that of any narrow- or wide-body twin.
Another possible factor - I suspect, can't back it up but it would seem to fit: 707s and DC-8s might have been overstrong at the design stage than more modern aircraft. Not suggesting that modern ones are flimsy, rather that everything possible has been pared down to the absolute known minimum weight, thickness etc. Half a century ago, when the first big jets were being designed, those minimums were a lot higher.