There's a lot found that very likely belongs to the airplane. It's just not a particularly sexy story. Most likely it's pilot murder-suicide, they did find a highly suspicious looking path in the pilot/copilot's flight sim. Iirc, during the handover from Malaysia to Vietnam ATCs, the pilot depressurizes the cabin, turns off transponders, turns around to avoid civilian radar, and once out of radar reach heads south across the ocean until the fuel runs out. Perhaps to have time to himself or maybe deliberately to not be found, out of shame.

It explains why the two main mysteries – why the initial search was a failure, and the lack of radio and transponder data.

It's also easy to see why authorities and airline operators want to silence it, especially if they have plausible deniability to do so. There is simply less prestige lost in a failed international search-and-rescue than a national airline pilot killing innocent people for god knows what reason.