The 3D V-Cache chiplets will cost more to make and it wouldn't surprise me if they have to bin them a bit better too(for less leakiness). It's entirely about cost IMHO(despite AMD trying to sell it as the best of both worlds),and the fact Intel is stuck at 8 performance cores. If Intel were to have a CPU with 16 performance cores,then expect AMD to suddenly launch an X3D part with both chiplets having 3D V-Cache.Either way,because AMD has not included a hardware schedular,it's at the mercy of MS,even moreso than Intel who has tried to include a hardware schedular.
The reality is that for gamers,the Ryzen 7 7800X3D is a better part,and for people running non-gaming applications the Ryzen 9 7950X3D mostly seems a better part. The Ryzen 9 7950X3D seems neither here nor there TBH,because especially if you are buying it for the point of moar than 8 cores for gaming,its going to hit issues due to heterogeneous core usage! It's telling that AMD is only allow reviews of the Ryzen 7 7800X3D weeks later.
I really wish AMD just launched two 3D V-Cache SKUs,ie,the Ryzen 7 7800X3D and the Ryzen 9 7950X3D with a homogeneous cache configuration.