1) It's not Nissan.
2) Most people are going to swap in another straight-6, V8, or turbo-4. V6 is the redheaded step child of the performance world.
3) The VQ engine is also readily available, and is way easier to find a manual transmission. Unless there's some other non-stock transmission that bolts up to it, I don't believe there is a manual transmission for those engines (I could be mistaken)
4) The MR2 community uses them because it's a transverse engine. The 240z requires a longitudinal engine. Not sure how much work would be required to make it so.
5) (And ultimately the biggest and only real reason) Because there are easier options out there that will achieve the same goal. The only reason I could see for ever doing that swap would be because you got one of the engines on the extreme cheap (free) and you wanted to just be different for the sake of being different.
That all being said, if you got one, go for it! I'd love to read about it.