The saddle stitches are inherently strong, since the threads cross to both sides of leather pulling the leather evenly. Since these are hand made, the stitching can use thicker threads to create perfectly uniform stitch lines that would last as long as full-grain leather.
In comparison, machine made lock stitches use thinner threads that could fit in the sewing machine bobbins. Since the threads stay on same side of leather they cannot be pulled with same tension causing the 'locks' to be uneven within leather layers. The entire stitch length could unravel easily when the thread breaks in any of the stitches.