With the analysis of the missing flight by commercial pilots and the emerging data, the reason apparently looks like a severe electrical fire aboard the ill-fated jet.
All the data obtained so far points towards a catastrophic electrical fire too. When an airplane encounters electrical fire in flight, it's a standard practice by the crew to turn off all electrical circuits including the location transponders and other stuff that helps controllers at ground track the plane.
It's a known fact that the pilots increased the flight level from 35000 ft to 45000ft. This again points to the electrical fire theory, as higher altitudes helps prevent the fire from spreading as the oxygen content at such higher altitudes is considerably thinner.
When the pilots noticed that the situation was becoming desperate they turned the plane around for the nearest airport. I doubt if they would have headed towards Kualalumpur as the prime concern for the pilots in such an emergency would have been to head to the nearest available airport.
We don't know yet but may be the electrical fire would have disrupted or damaged the cabin pressurization panel (similar to what happened aboard Helios Airways Flight 522 on 14th August 2005), and the entire crew and the passengers would have slipped into unconsciousness due to lack of oxygen before the crew could act any further. Meanwhile the plane would have continued on it's new heading till it ran out of fuel and crashed into the ocean.
The non-surfacing of the debris could be due to the extreme weather conditions encountered by the search teams near the approximate crash locations. Also, the debris by now could have been shifted.
Whatever be the reason this incident is shockingly mysterious and the entire world is anxiously waiting to know the reasons behind this mysterious disappearance. Prayers with the families who have lost their loved ones!