In the case of Campagnolo ends, they're a direct descendent of the toothed Cambio Corsa/PR ends, the transition away from which started in 1953 with the Gran Sport derailleur. A practical reason for them also dovetailed with the introduction of the Gran Sport--sometimes the "drop paralellogram" derailleurs benefit from slight adjustments in wheel placement which serve as a small adjustment in chain length. Think of trying to persuade a Nuovo Record derailleur to shift the chain to a 28 or 30 tooth cog--manipulating wheel location is one way to make it happen and isn't possible with vertical dropouts.
David Feldman
Feldman's Bicycle Repair
Vancouver, WA
360-694-4228/503-998-7249