Horizontal dropouts were necessary back in the day because of fixed gears.

When things went freewheel and multispeed, there wasn't a good reason to ditch horizontals. Campagnolo even came out with a derailleur setup that required that you loosen the rear QR in order to shift. The chain remained on a straight run and the wheel would slide in the dropouts to take up or provide slack. Tighten the QR down and you've shifted. Voila. It wasn't terribly popular as I understand given that there were other--what we would call more conventional--multispeed setups available.

Eventually in the mid-to-late-80s, for the reasons above, the industry recognized that there was some marginal usefulness to vertical dropouts and not much reason to keep horizontals.