Lack of any physical evidence. All the circumstantial evidence we have says it should be on the sea bed around where they're currently searching with the AUV, but nothing's turned up in the right place yet, on the surface or under it.
I strongly suspect it's down there somewhere, but, until something is found, people will continue to consider other possibilities.
What if the plane broke up shortly before entering the water?
That would account for the ULB pings (the black boxes are there but not the rest of the aircraft), the satellite arcs (the rest of the aircraft is nearby but just outside of the narrow search area defined by the ULB pings) and the lack of debris field (if the breakup occurred at a low altitude).