While the fanboy assumption seems to be that developers desperately want to work with the most powerful hardware, I think the reality is that the Wii U and "XBox 720" are (potentially) so similar in processing power because their consultations with third party publishers and developers indicated that they didn’t want cutting edge hardware.
I'm sure there are probably some teams that desperately want to work on hardware that is 16 or 32 times as powerful as the HD consoles, I suspect most teams want to keep development costs and timelines at (roughly) the same level they currently are; and this would be (almost) impossible in the middle of a graphical arms race that is driven by the release of new (super) high powered consoles.