Ergonomics
The face-down swimming position allows for a good range of motion of the arm in the water, as compared to the backstroke, where the hands cannot be moved easily along the back of the spine. The above-water recovery of the stroke reduces drag, compared to the underwater recovery of breaststroke. The alternating arms also allows some rolling movement of the body for an easier recovery compared to, for example, butterfly. Finally, the alternating arm stroke makes for a relatively constant speed throughout the cycle.
Read more about this topic: Front Crawl