By the looks of things, NVIDIA's Kepler graphics processing unit architecture has been such a success that the company doesn't mind stretching it for another year.
Originally, NVIDIA was going to launch the next-generation architecture in 2013. Called Maxwell, it would be both stronger and less energy-hungry than Kepler.
Due to TSMC's decision to only make 20nm chips in the fourth quarter of 2013, this will no longer happen.
We suppose NVIDIA could rush things and officially release a Maxwell-powered GeForce GTX 780 before the end of next year, but it makes more sense for the launch to take place at CES 2014 (January) or CeBIT (March).
That said, the GK11x family of GPUs will keep the market occupied for the next twelve months. This is what the newest roadmap slide suggests anyway.