Because most retro handheld users do not want to manage parent files. You want to put sf2.zip (Street Fighter II - Champion Edition) on your SD card and have it run, even if you deleted sf2.zip (the parent, World Warrior). Non-merged gives you that freedom.
Mame 2003-plus Reference Full !new! Non-merged Romsets Download
Because most retro handheld users do not want to manage parent files. You want to put sf2.zip (Street Fighter II - Champion Edition) on your SD card and have it run, even if you deleted sf2.zip (the parent, World Warrior). Non-merged gives you that freedom.