Two reasons: large, phone sized sheets of sapphire are difficult to manufacture and be optically clear. Also, sapphire is more brittle than glass... so despite

Two reasons: large, phone sized sheets of sapphire are difficult to manufacture and be optically clear. Also, sapphire is more brittle than glass... so despite scratching less easily, it's much much more likely to shatter if you drop a phone. Glass, as much as it has deficiency, is a compromise inbetween. And it's way cheaper, to boot.