Because Americans typically value straight line acceleration above all else unfortunately. Many don't understand that test track #'s don't even begin to scratch the surface when it comes to how fun / rewarding a car is to drive.
It's easy to sell numbers because people understand numbers but it's harder to put into context and thus sell the truly important aspects of a car that make it great. So what happens is that people wrongly / ignorantly assert the quicker / faster car is somehow by default the superior car.
For example, the MX-5 is world renown for being one of the best cars to drive on the planet regardless of price. Most Americans are for whatever reason more concerned with 0-60 times, etc so they'll skip the MX-5 for something which is quicker but not nearly as fun / rewarding to drive. Also many are really concerned with "image" in how other people will perceive them. Yes it's sad but unfortunately true.
As far as making the Z better there are many things they could do. The problem however is in convincing people slower doesn't mean worse. Because as I stated people still ignorantly assert quicker / faster is somehow by default superior.
With that said a good start would be to dish that huge 3.7l V6 which is basically maxed out for something lighter, more refined, etc. Maybe a 3.0L N/A 6 which produces around 300hp. This could help in achieving a closer to 50/50 weight distribution as well. They could also look for ways to shade weight here and there.
The brakes would perform better with brake cooling ducts. Recaro seats such as the ones in the EVO X would be nice. Other than that they could fine tune the suspension for better feedback and improve outward visibility. The car could be made superior without turning out to be faster.
The value of a car should be measured by how it feels from behind the wheel not its numbers so this idea that Nissan would have to build a lower powered version of the Z to compete with the BRZ / FR-S doesn't make much sense.
---
I am here:
http://tapatalk.com/map.php?rma3fk