It's because bitcoin is more known in the Internet rather than the offline stores. Digital goods is much convenient when you're doing business with bitcoin because both of you can send the item instantly. Bitcoins sends in an instant, and a digital good is sent in an instant as well. Besides, what else is going to accept bitcoin first, of course the people that sell stuffs online.
I doubt that offline stores will start accepting bitcoins because it will be considered as a way to avoid tax by the government. Although of course it still depends on the country that you're in.