Because it is the red headed step child in the Miata family.
Mazda didn't have the money to develop a ground up replacement for the NB. The NB was old in the tooth when it was replaced. They already had the well developed RX-8 chassis so they shortened it and made a Miata out of it. The lack of a clean sheet of paper
design showed in the cars diluted and relatively numb driving experience compared to the NA/NB. First year sales were good but not great. Sales were flat after that with virtually all the cars being sold to non sportscar fans with automatics. The NC hung around much too long
as again, Mazda was scraping pennies to build a replacement. Badasses that Mazda folks are, they stuck to their guns and gave us the ND which is every bit the ground up, bold, clean sheet of paper design Miata fans were begging for. The ND drives like the NA6 which is arguably the most "fun" Miata, albeit the slowest.
The NC is a good car. With the right mods, it is very good car. In no variant is it a great car however. Never touched my soul the way the NA, NB, C5, C6 and AP1 do.