Does anybody know where the aircraft was last seen on radar? Conventional radar drops off after the shoreline, I forget how many miles but it's not very far. If my memory serves me correctly, long over water flying is done on tracks and aircraft are "tracked" via sat com position reports from the pilots. It's been years since I've done it but I remember you are all by your lonesome as far as air traffic control radar is concerned.
If the terrorists want to make the plane look like David Copperfield made it disappear, over water leaves everyone guessing about it and it's contents whereabouts.
Sad for those families. My family unexpectedly lost a cousin for several months before his body was found and it tore my aunt and uncle to pieces wondering.