Because the car is absolutely a modern take on the original FGT and to a slightly lesser extent, the Gen II FGT.
Both came with V8's.
I feel certain 95%+ of their prospective buyers will want a V8. With the Aluminator crate engine (from FoMoCo) coming in at just under 600hp naturally aspirated, I would think that would be the obvious and most desirable power plant.
Has nothing to do with the abilities of the TT V6.