Because they're fed PR announcements that come directly from Nintendo. The actual E3 showing was a confusing mess. I get what they were trying to do, but they did it poorly, focusing far too much on a controller which could have been easily mistaken for a Wii peripheral, and all of the hands-on demos not only looked like Wii games, but in same cases appeared to be modded Wii games (looking at you, Super Mii Mario whatever).
They showed not a single game (no, a rushed demo reel compiled of footage from other platforms doesn't count), and spent not a single moment explaining what the console was about. No details on the UI, online, connectivity, or graphical output beyond a ho-hum garden tech demo. Hell, the name itself, 'Wii U', alone failed to differentiate the system from the Wii.
Yes, for anybody paying attention and 'in the know' it was clearly a new console. But the idea that this was obvious to everybody is absurd. I was asked by several people who were casually paying attention to E3 what the deal with the Wii U was, legitimately confused as to whether or not it was a controller peripheral or revised Wii, or a whole new system entirely. Most of them, when quickly looking for info, found nothing except a bunch of pictures of the controller and the Wii look alike tech demoes.
Nintendo revealed a console, except the console forgot to show up.