I am going with the line in the article about the fragility of the sapphire, it simply does not take impacts as well as glass especially in the sizes being used. Kind of like how yields are with semiconductors, getting this large requires much better processes than they have now.

I am simply glad we got past the use of glass on the backs of phones like the iPhone 4 generation had. All that did for me is have me learn how to swap the piece myself.

I think glass breakage is pretty low in the issues people have with these phones, weight and battery life are bigger issues.