It's a hell of a lot easier to find something floating on top of the ocean than it is to locate something that's sunk beneath the waves. Plus those on the yacht activated locator beacons, they were desperate to be found, while whoever was piloting the plane switched all such instruments off. Or aliens took it.
Also, it's floated to where it is now. The aeroplane could have gone anywhere. If something's floating you can track its movements - movements are a lot harder to track in air (because it's in three dimensions) even before you take into account the fact that the plane's got a jet engine, so it can get a lot further in a set amount of time than a boat. Also, for the plane you're looking at a much bigger ocean. There's a much greater possibility that the plane would have been found had it been packed full of American and Brits (if that's your point) though, I'll give you that. (Apologies - had a couple, so I'm not typing particularly well. Think I got the salient points over though.) Edit: 400th post! Glad it was a good one.
