Cost, I'm guessing.
This site lists an OEM GT-R transmission for over $10,000
Nissan OEM GT-R Complete Transmission G6 - Nissan performance parts
I don't know the exact price of a transmission for either the 7at or 6mt on our Z's, but I'm willing to bet that it isn't over $4000.
I've always heard that dual clutch gearboxes are crap at low speeds too, although I never encountered anything odd while driving a dsg equipped gti.
I'd love to see a dct on the next generation Z though, if they can keep it under a $3k option.
To add to what LakeShow said, the 7AT shifting in the Z is a lot faster than any other traditional automatic I've driven. Vs the G, it seems to rev match more, jumping higher on downshifts.