Your 12v dc adapters are providing two connections: positive/negative. One of those can be in the middle/center, and the other on the outside with some kind of insulation between the two contact points. It's fairly easy to implement that.
Additional contact points that USB would require would make it much more difficult to keep the connector round. You could attempt to extend the concept with 3.5mm jacks common with headphones to add additional contacts, but as pcgeek11 points out, you'd have a problem every time you plugged in or unplugged the connector because of power going across contacts where/when power wasn't meant to go across those contacts. With headphones, you might hear the sound of that power going across in the left and/or right headphone on your ears. I'm sure we're all familiar with that little pop sound as you plug or unplug your headphone while wearing them. For headphones, it's not particularly damaging. The electrical current passes across the wrong contacts for a brief moment, and then once fully inserted the power goes through the cable as intended.
For something that's data corruption sensitive, you can't have those misc "pops" of electric current going across the wrong connection points.
Edit: The ipod shuffle cable looks like TRRS on the 3.5mm side. So 4 connection points. I don't know for sure, but I'd guess that the 2nd R (Ring) and the S (Sleeve) are the power leads as they wouldn't go across another connection point when being inserted other than what it was intended to go across.
The T (tip) and the 1st R (Ring) are probably for data (my guess).
They'd have to make a TRRRS (5 connection points) for USB2 and TRRRRRRRS for USB 3. You would almost certainly have to increase the thickness from 3.5mm to something larger to accommodate all those R (Rings)... and it'd probably need to be different in size than 3.5mm or 1/4 in size to prevent someone from inserting something where it doesn't belong.
With the TRS concept, I'd be curious how that would affect data transfer as you'd likely not be able to keep every wire the same length in that huge TRRRRRRRS, and my understanding is that a lot of data travel is reliant on all the cables being the same length... especially at high speeds.
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