Two reasons:
1. The NC/ND Miata crowd is a much older demographic, keeping their precious car in the garage over the winter, keeping it bone stock, bringing it to the dealership instead of working on it themselves and only taking it out on sunny days. I don't click with that crowd.
2. The aftermarket support for the FR-S/BRZ/GT86 is completely overwhelming. Want a completely different exhaust setup or want to run flex fuel? Turbocharged? Supercharged? There's quite a few choices to select from. ND Miatas have some selection, but not nearly at the same cost or availability.
You can get rid of that torque dip quite easily if it's bothering you, by the way. Just need new UEL exhaust headers and a tune. You'll even make a bit more power when you're finished with it, so win/win.
Personally I'm loving that people aren't buying them because it's been keeping the barrier of entry way down. They truly are remarkable cars... There's a lot more to a car than the numbers. The suspension/geometry and chassis with 55/45 weight distribution, MacPherson struts, Torsen limited slip differential comes standard, and low center of gravity due to its boxer engine are top-notch compared to something like a 370Z.