Except you're wrong OP, most laptops don't consume 2-3 times as many watts per hour.
You're looking at it the wrong way anyway, the reason it uses so little power is because it's not using cutting edge CPU/GPU so the power demands are much lower.
There's not a hope in hell something even using this "low" amount of power could be made into a portable for at least another 5 years, the Wii-U is already built off what 28nm processes? Certainly around that figure, die shrinks have really started to slow down it's going to be literally years before it could be put into a smaller shell with a lower TDP.
Also, you have to take into account that doing such a thing would be pointless in terms of profits for nintendo, it would be a massive long term project with very little interest...oh look a Wii-U in 5 years time inside a DS case...in 5 years, that won't be cool.
Talking in "now" terms, 39 watts an hour is a lot of power in terms of putting into a portable, that would be 2 hours on a standard laptop battery weighing just under 1KG.
Would you consider that portable and that is excluding a screen!!!?
Also, the technology in terms of power use used in the Wii-U is already cutting edge, that's the benefit of not using cutting edge in terms of actual performance, some very efficient designs can be churned out, which again counts against pushing things forward in die shrinks - as we're already at "the best".
