The major reasons why not a shred of wreckage has been found, are:
1. This is the first B777 out of over 1100 built that has crashed into water - anywhere. Thus we have no real comparisons to figure out how a B777 might behave when run out of motion lotion and then ditched. They're a very robust aircraft, and trying to compare the ditching of the Airbus A320 into the Hudson is trying to compare apples with oranges.
2. I think few very people have much understanding of the sheer vastness of the Indian Ocean. This is an Ocean where you can fly for 10 hrs at 550-600mph and not see a single solitary piece of land - and very few ships. Most shipping travels between Fremantle and Singapore, Japan and China - and only a relatively few go from Fremantle to the ME and India and Europe.
3. If the aircraft ditched in the current search area (as this is still the highest possibility of all the scenarios), any wreckage will have been driven by prevailing winds and currents in a Westerly, or Sth-Westerly direction.
Some permanently buoyant wreckage will possibly be merely circulating in eddies in the mid-Indian Ocean, far beyond any current aerial search area.
Possibly 50% of the initially-floating wreckage will have sunk by now.
In time, perhaps in weeks to come, some small pieces of identifiable wreckage may appear around Mauritius, Reunion, Madagascar, or the Seychelles.
I'm personally amazed that no reward offers for MH370 wreckage, to fisherman in the regions surrounding the Indian Ocean, have been made, to assist in wreckage recovery and ID.
4. I'm of the personal opinion that the Bluefin-21's capabilities are being stretched, and that better equipment is needed.
I feel that dedicated sea-floor mapping ships would be better suited to the search for the wreckage, from here on in.