My thought on why frames have gone to vertical dropouts is because the modern bike buyer wants to look like a racer thus they buy racing frames. Manufactures know this, so they had to tighten up the frame geometry, and if they installed horizontal dropouts you wouldn't be able to get the wheel off due to making contact with the seat tube. My Trek 660 is an example of that, though it has long horizontal dropouts I have to flatten the tire then press the tire against the seat tube to remove it; if it had vertical dropouts it would be easy to remove the rear wheel without having to flatten the tire first.

But before anyone shoots me on those comments, I'm just guessing.