For two reasons I believe:
1.) One reason I think they do it is they use it as another incentive for the consumer to want to purchase the more expensive model. Similarly to why they use the sapphire display on the stainless model, which they probably could use a sapphire display on the sport model if they wanted to. But Apple wants to give the "Better" features to the higher end models.
2.) Also, I think in order to keep the price point Lower for the entry level sport watches, they use cheaper alternatives such as the Ion- X Glass and composite backing. If they implemented the most expensive features like the Sapphire/ceramic backing, then That would also increase the price point.