Several reasons for this.
One, and among the most obvious would be "revenge", a lot of people are still sore after the Wii avalanche that took the market and buried the HD consoles and these doom threads are a sort of payback.
Another one is the relative justification for such threads, the overall expectations for the Wii U were fairly low and yet it has failed to meet them and it seems impossible for Nintendo to reach their fairly conservative shipment goal for the first fiscal statement. People like to cite how it beat the 360, which launched with no good software, massive hardware faults, a higher price and came off of a completely dead console with hardly any impact on the industry (from a manufacturer with no console pedigree what so ever), hardly the recipe for an obvious winner. They also go on to laud it for beating the PS3, which launched in february, a slow time of year with little to no software releases slated and at a crazy 600$ price point, it was also the very last console of the three to make an appearance, unlike the Wii U (and 360) which came first.
Another big contributor is negative press, the Wii U is riddled with smaller and bigger issues that bother the users and testers, slow loading times (really, really slow), sluggish updates, lack of implemented features from launch, poor storage solutions, few games which take into use the Gamepad in any meaningful way, lack of 3rd party commitment (old ports do not count as commitment) and exclusives, developers reporting in on a very slow processor and slow RAM etc, the list grows long and is bound to make an impact, especially with potential early adopters, which will typically be more tech savvy and more likely to read about it in the first place. It has also gotten a fair amount of flack in mainstream media. Launching unprofitable hardware is not an advantage either when you live solely off of games and gaming.
All in all, the Wii U launch has been a poor effort by Nintendo, widespread reports of stores not assigning dedicated shelf and floor space for it, people not understanding what it even is, practically no advertising for a consumer electronics product which launches at the busiest time of year, the above mentioned problems (one could almost say that they launched an unfinished product). The whole process reeks of stress and pressure. It has launched with a behemoth title and a stake in the biggest multiplat franchise of the generation (New Super Mario Bros U and Call of Duty) and has even gotten franchises like Assassin's Creed, the price is reasonably low for a launch console, it comes off of a supreme market leader and a company that has held the handheld throne since it was built, it has launched in the most favorable time of year possible and has had a good assembly line and few supply problems (unlike the 360 and Wii, for instance).
The Wii U has done okay so far, not horribly and not great but okay, all things considered. January, february and the summer months will be very telling. I wouldn't say that it is doomed but I do think that it could be in for some very early trouble. As a last note, to everyone saying; "Hah, oh yeah? Look at the Vita then! What a flop!" Resorting to that in and on itself proves that there is some trouble afoot, seeing the well and truly atrocious sales it has managed thus far.