ZombieBBQ's post is pretty much what I gleaned from GAME's woes
I am surprised they got suckered into believing the Wii was a long term thing, I love my Wii but even from the start it was plain to see it's mass popularity wouldn't last forever. So many tales of people scrambling to get a Wii for Christmas 2006/7 - playing Wii Sports and whatever else they had bundled with the system (Carnival Games usually...) and then it never got used again. Nintendo kept some casuals going with the release of Wii Fit and a few other games, but that was all they could do.
They handled the DS fad a lot better, it got some preferential shelf space, but they made sure the PSP was well supported for a while and still had fairly fair shelf space for all systems then. All systems got fairly well treated, but at the end? Good luck finding a 3DS title in the DS rack or a new Wii game!
Quite why GAME didn't twig when Wii sales started to drop and Activision themselves actually called time on the plastic accessory party I don't know (though in that case, I am rather surprised that GAME wasn't more supportive of Skylanders, which made a killing over the holiday season)
Though I assume the inability to stock certain special editions, 3DS titles and Skylanders was more to do with GAME's emerging troubles. It didn't all start with Mass Effect 3's loss or even Nintendo's early bowing out. I think it goes much earlier to around the 3DS's release. As it was interesting to note that GAME and Gamestation were a whole lot less keen to stock new 3DS titles after launch, some of that due to a lack of market confidence in the system, but after Ocarina 3D - they really dialled down support for the system. Which I think to a certain degree hurt the 3DS a little, Nintendo certainly had an ax to grind with them for that one. They stuck to the familiar horses they had put their bets on and stuck to it.
They rejected Sega's Sonic Generations CE too, leaving Amazon and any other stockist willing to take it on to enjoy the spoils. I don't know what the earning margin on that set was, but it seemed a more surefire seller than any other collectors edition last year (even if it costed silly money). Yet they passed it up other other CE's, which I'm sure sold well too...but seemed a lousy decision all the same.
Now Apples iOS devices and Android tablets are flavour of the month, though one would assume they are lasting longer virtually because the apps cost next to nothing. But only Apple and App publishers are able to benefit from that market. That said Gamestop US started trading in android and iOS last year and seemed to be making good money on refurbishing older models. They also had some kind of digital distibution model and a flash gaming portal, which seems to be getting money in for them.
Basically it's a lot like Borders downfall, another company who had a management in place, that seemed to sorely misunderstand the specialist market they were working in. "More media! Less Books! Less sitting space! GREAT IDEA"