You're right. But speaking from a competitive side, the competitors against the NSX are simply more desirable; that's why it's sales haven't been the best.
And S2000 is the only one that I really think is above average compared to the other Hondas. But while you can dump aftermarket parts onto a Civic or Intregra and hope it runs faster, this is true of any car. The only plus is that parts are numerous and cheap.
There are just easier cars to work with. But I guess it's simply a preference thing in this hobby.