Quote from ZeBedie:
"Which may explain why it's the only item to be found?"
Or, as 172driver and I have discussed above, because this flaperon - assuming it's from MH370 - happened by pure chance to be washed up on this relatively-tiny, well-populated island after floating on or beneath the surface for about 16 months.
Is there anything unique about the floating capabilities of a flaperon, as compared with other primary or secondary flight controls made of composites? Or, for that matter, belly fairings, landing-gear doors, fan cowlings, etc,? If not, there may be large quantities of other nearly-submerged pieces that missed the tiny island on their way west.
The reported theory at Toulouse is that the flaperon was the only part to detach from the a/c before the entire airframe sank to the ocean floor. It's been suggested here that it might have detached in a very high-speed descent. However, unless a pilot managed subsequently to regain control of the a/c and perform a textbook ditching in calm waters, there would be lots of debris. Some of that might still be afloat near the western shores of the Indian Ocean, or already ashore.