I think it's a lot of things compounded rather than one single reason. Horrible console naming is a brand issue amongst kids. Kids hitting 8 and up don't want Nintendo in their house. They want to play COD with their friends and care about it being a cool console thats powerful (whatever powerful means to them).
The first party lineup is rock solid but the tablet controller does suck as it weighs a ton and a lot of gamers were horrified seeing a tablet in their gaming device.
Nintendo also doesn't understand it's customers at times and pushes so much for innovation that it's gotten a knack at alienating it's consumer base. The sheer volume of people I knew who went 'I'm done' when the Wind Waker was announced massively harmed the GC console, and while WW is one of my favourite games of all time the look switched off the mainstream. The same thing happened again recently when a couple of my friends loved the Zelda 2012 trailer and were going to get Wii U's when the realistic Zelda launched. They'll never get it due to the new ones looks. (Personally I can't wait for it)
Nintendo until they switched to the direct E3 format could have an amazing E3, or be the worst show of the big three. During the Wii days a lot of damage was done to them trying to flog garbage like Wii Music and casual games while the core audience withered away.
I love Nintendo, they make the best first party games for me personally, but they have only in the last few years started to accept their issues. The Wii U itself was not enough, they tried to cling onto the casuals with the name, the gimmick controller, launching Nintendoland with it and underpowering the machine and they're paying for it big time now.
