This photo aptly demonstrates why. That's Robert Scoble, respected... erm... tech news nerd, wearing Google Glass in the shower. Not enough to stop you ever buying a pair? Have a look around the web, there's just not a situation in which Glass makes you look anything but ridiculous.
Most people won't even consider a bluetooth headset, can you honestly concieve of them trying this for size?
Hopefully that doesn't kill the Glass. But Google needs to go back to the drawing board to come up with a product which doesn't look so irrevocably ridiculous.
And once its done that, it might be an idea to give the first devices to the beautiful people, the trend setters and the pied pipers of fashion. There's a reason why these people are used for selling fashion, because we (the great unwashed) need to aspire to be like the person using the product to create a desire to buy it.
As far as I am aware, nobody in history has ever aspired to look anything like Scoble, in the shower, wearing Glass.
And that's what kills products...