Because Nissan says so. You get some minor changes including a new grille, an improved interior, and a larger touchscreen. Power and torque remain the same at 600 and 481 lb-ft respectively.
Nissan apparently is bridging the pricing gap between the GTR Nismo and the Porsche 911 Turbo S but the GTR was respected for value and the GTR does not lend itself to price hikes at the end of its lifespan.
If you are considering this car do yourself a favor and buy a used GTR or just a standard 2017 GTR, tune it, and laugh at anyone dumb enough to waste an extra $100k+ on Nissan's aging robot.