Any reason for the style of rear dropouts

If you have no rear brake, then track ends are the simplest, easiest solution.

As soon as you add in considerations like a rear brake and mudguards, then horizontal dropouts are infinitely superior. Especially for brakes where the angle of the dropout aligns with the brake pad so that moving the axle position doesn't result in a need to realign the pad to the rim.

So horses for courses.

Bang a sensible answer! Depends on the use of the bike. the preferance for trackends at the moment is partly fashion..