Speed of sound, you may be right about it being easier to rob the truck but if it was 4300kg of gold you were trying to pinch (see earlier posts for derivation of this figure) how would you move it? As bizarre as it seems, taking the plane is logistically easier, and, as may have been proven, more likely to succeed.
Not saying this is what has happened, but until the cargo manifest is released this option is on the table.