My guess is, they had an electrical short/smoke/fire, which disabled both their radios and transponder. It seems they did initiate the initial action items as we are trained; turn towards your divert field, descend below 14,000.

It seems after they accomplished those two things, they were subsequently overcome by the smoke/fumes and the airplane continued on autopilot, holding heading and altitude, as it should, until it ran out of gas.

In the UPS 747 fire and crash in Dubai, their pilots got their oxygen masks on, and their smoke goggles on, but the smoke was so think they could not even see their instruments, from less than 6" away! They made a low pass over the Dubai airport, but were going 250kts when they did. So they tried to go around and do another approach, which ended with them crashing into the desert just outside Dubai.

If you can't see your instruments because the smoke is so thick, and you can't ventilate the smoke, you can't fly the airplane. You are only going to live until your 02 runs out anyway, which I'm sure was far less than the 7 hours MH370 flew on autopilot before it ran out of gas.

Here's another issue that might arise if/when they ever find the **** Voice Recorder (CVR). In the old days, the CVR was a cassette tape which ran in a 30 minute continuous loop. When the 777 came along, Boeing went to a digital recorder, but from what I've been told by my simulator instructors, it only has a two hour loop. IF...they were fighting a smoke/fire situation, lost the fight, and then the airplane flew on for more than two hours, the CVR is going to be blank, as in no voices at all. We may never hear what they were talking about when the problems started.

The Flight Data Recorder will have a lot of data such as airspeed, altitude, heading, systems which were inop, (Hydraulics, fuel on board, etc.) I don't know if it will tell them definitively if there was smoke, or where the smoke was coming from, unless the FDR also records which electrical busses were lost due to the short, etc. If there were some other reason for the smoke, like somebody was intentionally starting a fire, or a small bomb/hijack scenario, it won't tell us that.