Actually, the Xbox has never been 64 bit.
And to answer NSY's question, NSM already explained it, but I'll now re-explain it in my own words:
3DS: Lots of great games, that can only be found on that platform. High priority from 3rd Parties.
Smartphones/Tablets: Countless of people playing on them everyday. It's the future for Indie Developers.
PS Vita: Attention from 3rd Parties and Sony once in a while, almost no exclusives due to Cross-Play. And lots of PS3 Ports make it not worth getting.
Wii U: Started with lots of games, almost nothing happened shortly after. 3rd Parties rather stick with the PS3 and Xbox 360, due to the current Wii U sales. Nintendo is still hiding lots of new stuff for after the E3.
PS4: We know basically nothing about it, other than how the Controller looks like, and the fact it has the same specs as a High-End PC. Nobody knows if this support from hundreds of Developers is true, or just a broken promise.
Xbox One: It's a Console that has a bigger priority on Multimedia, than Games. Ships with an OS that's more suitable for a PC, than on a Console.