because the 300ZX twin turbo is a V6 and not a Straight 6....
and the engine bay of the 300ZX twin turbo is soo compact, to fit a single turbo would be very very expensive... ehh...
so that guy who did the single turbo conversion.. has lots and lots of money
So it's cheaper and more affordable for a Supra.. then a 300ZX...
but ehh... both kick ass machines