So that the bow side of the shackle can slip through the hole. This allows the shackle to swivel and rotate if side-loaded from a pull to the side. If you put the pin through the shank you'll need to have an oversized link welded in the end of the chain (or use an undersized shackle) and the side load would allow the shank to act as a lever to spread the ends of the bow shackle apart.