Was the stuff simply too fragile in large sheets?
Until someone manages to make single crystal sheets 150mm in diameter, it's hard to know what the real world properties are going to be like. There is a big difference between a piece of crystalline alumina 30mm in diameter and fairly thick, and one 150mm diameter (and possibly rather thinner to allow for an effective touchscreen).
An historical example would be the big gap between the development of wrought iron, basically a craft skill, and the open hearth furnace.
One MD I worked for remarked that the person he would most like to have been was the Pilkington - not a family member - who brought about the first Pilkington float glass plant. Because you can't build a prototype - it has to be big enough actually to make float glass, and that means that, just as with a Lunar human-carrying lander, the first prototype is actually the production model. Did Mr. Pilkington sleep at night while the money was being spent and the factory was going up?
It's possible that something like this has happened here - that the sheets just don't have the right properties and it may take years of development to make it happen.