The X360 and PS3 were marketed as next generation based upon their capabilities, not their release window. The Wii wasn't marketed as next generation but rather based upon its innovative controller. The Wii marked the first time a major console manufacturer opted against major spec increases in favour of targeting the casual market, as in hardware terms it bore little relation to the competition. Next generation implies that it is beyond the last generation, which the Wii U simply isn't. I'm far from the only one who doesn't consider the Wii U to be next-gen.
Nintendo has a very different strategy to Microsoft and Sony - it simply isn't trying to compete on power or mature games, which makes comparisons more difficult.
Nintendo mightn't like it but a lot of people are going to assume the Wii U is part of the next-gen and think it is a competitor to the nextbox and the PS4. The same thing happened with the Wii.
And the PS3 was supposed to be more powerful than the X360 yet that never materialised. It always comes down to market realities and the poor sales figures for the Wii U certainly aren't helping things. Why would publishers put such effort into a version that, at best, might sell 1/10th the amount of the X360 or PS3?
Don't bother. Blackhearted think s the reason why more games aren't being ported to the WIi U is because devs are "lazy." Economics apparently have nothing to do with it.