Opaque ones have a wider range of materials to choose from. Plastic is pretty unlikely to break, though. Glass is the only other transparent case and that would be brittle (guess which one modern phones use)
You know, it'd be kinda sick if someone made a device that had a colored glass case. Phone glass would probably still be too fragile to make an entire Gameboy or iMac out of, but it might be cool at least as a concept item.