It's not that people don't think the 8th generation has arrived definitionally. It's a disagreement that the WiiU is functionally 8th gen. Just that the WiiU itself isn't a large enough technological step to feel like it's 8th gen technology. It's more powerful than the 360 and ps3, yes, but not by enough to make a drastic difference whereas the consoles we're getting next month are something like 10x as powerful (ps4/ps3 comparison).
What we like to see from generation to generation is a significant step. Thta is the entire point of dividing consoles by generation, they're tiered. Something large enough to make us want to change and to give us a new experience. Nintendo singlehandedly brought peripheral control to the console market with the wiimote. While technologically everyone laughed at it, it at least made a step in user interface that made it fresh and enjoyable. This time though, it uses the same controller and a gamepad which only a handful of games actually utilize and the ones that do generally have workarounds that would manage on a thumbstick. So for an added $140 price to the console that is an underpowered entry for the 8th generation it's no wonder why it is dismissed.
Add that to the fact that the WiiU isn't popular at all. It's selling around 120k units per month now (1.5 million units per year to put that in perspective. They wouldn't breach 10 million for another 4 years at this rate if we assume they've hit 4 million this month). So, they are on track to sell fewer units than the Dreamcast did in the same amount of time. Considering that the gamer market is larger now that means it's doing even worse even if it exactly matches the Dreamcast's numbers (dreamcast sold just over 10 million units from september 9, 1999-march 30, 2001. (first launched in Japan in 1998 but didn't make it to a global market until sept. 1999). 19 months in the global market, 10.4 million units. That's 528,421 units per month on average. Even giving it the full 30 months for the year it was in Japan only you'd be looking at 334,700. WiiU's average is around that number thanks to the initial five months that saw 3 million units sold. But with every month that ratio is dropping and next month it may be exactly that rate as it finally exceeds 4 million like it's so close to doing. But it has to be over 5 million per year to even match the Dreamcasts' sales. Sales should have been much higher last month. The big $50 sale hit on the 20th and we only saw 10k more units sold in September than August. I didn't expect the $50 to fix the problem, but I did expect better performance.
So while yes, this is the 8th generation, it's rapidly shaping up to be one of the failures of the generation and not a legitimate entry. Nintendo does have those from time to time. Consider that the gamecube only sold 22 Million units when all was said and done and I believe it was even more powerful than the competition despite being cheaper. Playstation 2 sold nearly 7 times as many. Followed by the wii which has just broken 100 million. So while the WiiU may fail, Nintendo isn't necessarily out of the game. But this does certainly make consumers not feel like the console is a legitimate 8th gen entry. The dreamcast was also the system that followed the abysmal Sega Saturn. Successful console companies can usually wade out one or two failures after a successful generation. Nintendo is sitting on gobs of cash so this won't be the death of them. But it will signify change for the company. It's difficult when Nintendo doesn't have the sort of ties Sony and Microsoft have to hardware companies.
But for all these reasons, the WiiU just looks like a really late entry into the 7th generation without being advanced enough to warrant a new tier. If it is 8th gen, it is the Wii of the 8th gen but without a wiimote equivalent to push it.