I have an ORIS watch, sapphire glass, front and back, but to negate the fact the this kind of glass is brittle, the watch glass is rather thick. While in the ten years I have had the watch here is not a mark on the glass the case has picked up a number of battle scars. The watch is also quite heavy. Thin sapphire glass is all well and good and relatively light when thin but it shatters easily.
It's more than likely that Apple didn't use it in the design because it wasn't up to the job.