Originally Posted by brokenrobot
Real reason: In the old days, people liked a relaxed "touring" geometry, which allowed for horizontal dropouts. Now everyone wants faster, tighter geometry, which requires the wheel to be closer to the seat tube, which is impossible with horizontal dropouts (because there's no space to slide the wheel into the dropout - the wheel would hit the seat tube). So now manufacturers use vertical ones.
in addition to this, the world is on a big "user friendly" or "plug and play" riff right now. the vertical dropout has some virtues, but i think that the biggest reason it has become popular is that its a simple, brainless, layperson oriented feature.
see the rest of modern bicycle component manufacturing for further examples of this.