There's much more to a name than just what brand it symbolicly falls under. Marketing, selling and servicing for high-end buyers is a completely different business from moving Sentras during "Nissan's Deal-a-Thon Days."
Volkswagon famously floundered with the Phaeton for (among other reasons) giving $70k luxury customers the $15k Golf treatment. Their dealers were simply unprepared for the level of service and ass kissing that premium buyers demand. Customers were furious with the "tough luck" attitudes they routinely give the Jetta girls. The same problem afflicts Corvette buyers. They pay $55k premium dollars for their car and get the Malibu treatment every time they need service. It's a problem that Hyundai is facing now and it'll happen with the GTR too.
In this case, they had a perfectly viable upmarket dealer-base ready to service high end clients. Those dealers were also loudly asking for a halo vehicle like the GTR to solidify their slightly shakey upscale brand position. With the global/Euro launch of Infiniti coming up, this would have been a perfect move for Infiniti. But giving it to Nissan just p*ssed off their luxury dealers, compromised the vehicle's ability to command a premium price, and sacrificed both potential profitability and the opportunity to bring ultra-high end customers into the brand.