For years I wondered how the technologies for Damascus and Toledo steels could have disappeared for centuries. Watching how the most basic knowledge disappears in a few short years in the bike world, has shown me how.

As others have said, QRs have been used with forward opening horizontal dropouts from the their invention, until horizontal dropouts went out of fashion. Slippage was all but unheard of, and Men like Fausto Coppi could climb the steepest alpine passes without problems (mechanical) of any kind. If modern hubs slip, when their predecessors didn't even for Eddie Merckx or any of the Pro riders, we should be wondering why.

As Hillrider said, better QR design and construction is part of the answer, though I prefer to chalk it up to the lack of decent dentation in today's locknuts, and QR faces. Picture trying to hold a shaft with a pair of smooth jawed pliers vs, a pair with new sharp serrations. That's the key old stuff bit the dropout like a pitbull, and the new stuff doesn't.