ex-EV1 driver - The cost shouldn't be an issue. All of the internal wiring of the battery is already big enough to handle 6.6 Kw because the motor requires it. It shouldn't add anything to the vehicle cost.
The problem that led to the wimpy 3.3 Kw charging was simply a lack of foresight. Nissan product managers didn't think beyond Japan whose 240 outlets are limited to 15 Amps (~3.3 Kw) and didn't realize that US households usually are wired for 240 volt, 30 and 50 Amp service to handle electric dryers, ranges, and air conditioners.
I'm fine with Nissan using 6.6 Kw as a value-add option though. There may be some people who really don't do much driving for whom 3.3 Kw is sufficient, just as 1.2 Kw (120 volt) works for some. Choice is good.