For the same reason APS-C, APS-H and any single sensor lens/camera is. It doesn't force us to buy all new lenses every year, like we are now expected to do with bodies. The future is the plenoptic camera lens.
Photo industry braces for another revolution | Underexposed - CNET News.com
The reason Pentax/Samsung are on the right track is because they are concentrating on making the SENSOR ITSELF better, not concentrating on wasting time dealing with MANUFACTURING issues that are inherent in making larger sensors.
The future is multiple, small sensors, integrated with multiple small lenses where all processing is done in-camera. Moving to FF or MF is regressive, archaic, backwards and technologically antiquated. Looking backwards to design criteria (like FF or MF) standardized before you were born for FILM is folly. Buy NEW stuff man! Arguments for FF or MF are predicated on existing and/or old technology and assumptions that the laws of physics are paramount as they apply to single sensors, and single lenses capturing a view and NO digital processing. What happens when you have 5 overlapping sensors capturing the same light? 10 levels of dynamic range. No problem. Is that gonna happen with single sensors? Not at the same price! Sensor technology is getting better and better. With the new Samsung sensor, we are getting the same noise from a larger sensor. Do you really think that these types of improvements are going to stop. No. It's going to get better and better.
The K20D's new sensor has improved the APS-C sensor by reducing spaces between sensors. The camera itself has amazing in-camera processing capabilities. It foreshadows the future of where digital photography is moving, and where R&D funds are being spent - SMALL sensors, not big ones. Need lots of resolution? Well, it'll just be adding more sensors and processing power. A cell-phone will have one, or a few sensors, a hi-end camera will have dozens of the exact same sensor, and they won't be APS-C, APS-H, FF, MF or any film-derived format-size. It will start happening very soon if you also look at the new Intel chips. They're amazing. FF and MF advocates are living in the past. It's 2008! If Pentax/Samsung do release FF or MF cameras, it'll be for marketing reasons, not photographic reasons. You'll soon be stuck with a proverbial, over-sized lemon.
So today, buy APS-C from Pentax as they improve this sensor size, until they replace it with a multiple sensor lens 'system'. FF or MF, if they ever happend will be a short-term dead-end. If you "NEED" FF today, switch companies and stop posting on this forum. Soon, you will ask what kind of processor is in your camera, and how many lenses it has, not what the size of the sensor is. We'll be looking back at DSLRs like we now look at 70's muscle cars. Kids will make fun of you and your non-3D HUGE camera.