Knowing nVidia they probably realised that the 6GB 780 meant that people wouldn't have to upgrade to avoid VRAM problems. They always seem to give just enough VRAM for the current generation and nothing more.
Or they didn't anticipate the demand for these cards correctly and cannot keep up with demand so OCUK have pulled the listings to avoid people getting frustrated at ETAs being missed.
Or they didn't anticipate the demand for these cards correctly and cannot keep up with demand so OCUK have pulled the listings to avoid people getting frustrated at ETAs being missed.

