I have seen others answer the question of why this shape (aerodynamic efficiency) but not why it is aerodynamically efficient.
The key in producing an aircraft with high Aerodynamic efficiency(lowest drag) is lowest possible wetted area and frontal area while simultaneously generating maximum lift. Frontal area is just what you think, the lowest possible cross section that you try to push through the air. Whetted area however is the the area that gets "wet" ie the total surface area of the aircraft. This means that the best possible shape is a long cigar: the thinner the better. Any area that you add to the "tube" is wasted energy.
Simultaneously the wing shape that generates maximum lift with lowest drag is a long thin shape. This is because wings cause lift primarily by creating negative air pressure by making air flow faster over the top than the bottom. This effect is destroyed by having a wide, fat wing because the wing can't create the same tight pocket of air differential between top and bottom that a thin wing can. This is what kills Delta wings and lifting bodies.
Once the speed of the aircraft goes to a larger percentage of the speed of sound however other factors come into play. The wing must be swept and the wing must be stronger to handle the forces in play so it must be thicker.
The last portion is the tail in back. This is just mimicking the arrow: putting the fletching in the back of the arrow creates natural stability by making sure the front of the aircraft stays pointed forward.
There are many advantages to other designs, Canards have great stall behavior, lifting bodies have superb strength and therefore safety and tandem wings with struts have great strength and great lift characteristics. But the airline business is a high volume low margin business so any tiny bit of drag gets voted down when decisions get made.