Interesting about the sales lag
My understanding when they launched it was that it was intended to get families off of motorcycles, as it is not uncommon to see 4 or 5 people on a 125cc bike. It was deliberately made inexpensive so that those families would be able to ride in something safer. Perhaps $3000 was still too much for that purpose, and too little to be taken seriously as a car.
The Nano might do better in the U.S. than the Smart, simply because of the price. The Smart is priced higher than some much nicer and larger cars, so the only reason to buy it is if you never, ever, ever expect to be on the freeway. Then, the convenience of its small size might make it a better choice than a Mazda 2, Ford Fiesta, or Chevy Spark. If the Nano comes in under $10,000, it would fit with the "smaller costs less" view.