Several reasons. Some of which:
No seam that could potentially fail.
On a full sole all of the stitches are picked from the welt. That can't be done on a 1/2 sole. So, there will be double stitching where the 1/2 sole is joined.
The foot-bed is completely stripped and replaced with a fs. On a 1/2 it's patched together.
What are some of your reasons?