Well, normally the top end card has two variants. A base VRAM and a double vRAM variant. There are no 6GB 780ti cards available for those that happen to need the extra vRAM for their use case, so they're kind of forced into the Titan Black instead. I think a 6GB 780ti would sell decently well, but it would likely cannibalize Titan sales and there probably isn't room or need for that fine grained of a product differentiation in a market segment that small.