Power is proportional to the voltage squared (P = V * V /R), so the power supplies need less current -- cheaper, more reliable - less heat, less voltage drop.

The same holds true for the power supply in NA vs most of the world (save Japan). 110V vs 230V - much less copper needed for conductors. motor windings, etc.

Flip note: the better option is using constant current drivers, for higher efficiency (they might have unsafe voltages, though)

More light output mainly. If you have a look at the philips hue lightstrips you'll see it there too. The 24v pumps out way more lumens. i.e. Useful levels of light not just decorations.