Older skewers had serrated surfaces that dug into the dropout. The bikes usually had a soft chrome plating that permitted the skewers to dig in. This added friction and prevented the forward motion generated by the chain/cog interface.

Vertical dropouts resist the forward motion. It isn't necessary for the skewer to resist any forward motion in a vertical dropout. The skewer manufacturers were happy to eliminate an extra step in the manufacturing process.

Riders still using bikes with horizontal dropouts and have been skewered.