In particular, Apple's boss points to the immaturity of the display technology, which doesn't have a high enough quality nor field of view, as well as the issue of putting low-power hardware with enough processing power around your face:
Cook didn't address all of these technical differences in the interview, but he did talk about the prospects for AR goggles or glasses. He believes the tech isn't ready:Via: ARS Technica
The technology itself doesn't exist to do that in a quality way. The display technology required, as well as putting enough stuff around your face–there [are] huge challenges with that. The field of view, the quality of the display itself—it’s not there yet.