Economics of scale: probably. But maybe there are also other factors. For example it seems impossible to power a large container ship with solar cells, but maybe for a smaller container-ship it would become feasible? Some not so urgent goods could perhaps just drift in the currents, too? No idea, though...<p>I actually just had the other thought to call out a sort of X-Prize for the first autonomous robot ship that crosses the ocean. Or maybe it has already been done anyway.