I'd like to post an outsider's perspective on this issue. First, I'm not sure if you refer to slower sales these days or more generally over the years? Economic turbulence aside, the NSX seems to me rather under-rated, probably because of the luke-warm press coverage the car received over the years in North America. The mainstream automotive press never seemed to appreciate Honda's engineering approach based on balance, precision and refinement rather than overwhelming brutality which had been the essential sales point in the whole supercar segment (and the center of media attention).
There could also be an issue with the automotive culture which is slowly changing, people nowadays lust over different kinds of cars than gearheads did 10 years ago. The definition of a performance car evolves, the flashy space ship design which has been the exclusive attire of the fastest supercars has been challenged by the boxy likes of the EVO, WRX and now GTR. In this sense, for a portion of the car crowd, the NSX may be one of the greatest specimens of its time but belongs to another era. Performance nowadays comes in a much different package, not everyone buys into this new trend however.
Of all, there is no doubt in my mind that the NSX will be the most valued of the mass produced oldschool supercars. Once the economic uncertainty clears, NSX sales will accelerate, and turnaround time will shorten.