This was brought up recently on a thread about streaming content and the lack of at seat power. When you consider BA, LH, U2, FR, MON and WN all have new cabins and none feature at seat power it would seem that we are a long way off having widespread at seat power on short haul.
I would think it's because there's very little revenue gain to be had. I junk SCOOT charge for power access but I still can't see it being much of a money spinner. I know it's a train but OUIGO charge extra for seats with power on french domestic routes. So there is a revenue gain to be had. Wether it covers the cost of installing, maintaining and administering the system and could make a profit, I honestly don't know.
As for long haul, if there's already IFE and the infrastructure is there then surely it wouldn't take too much to install at least a USB.