Because cell phones are discrete fashionable accessories that serve as status symbols displaying varying wealth and disposable income, while Google Glass is a lopsided glowing deformity on the side of your head.
I donno. Add some options for racing strips, a hello kitty theme, some godawful gold stipple (a Dalek fashion, but popular), or whatever, and suddenly people will love it.
Remember how popular the iMac (~1999) was when they started adding colors and patterns to the outside?