His back is vulnerable

The reason the weak point on his back is exposed is because of a “curse” that forces him to do so. It conceptually prevents him from protecting his back. Even if Siegfried were to be protected by something like a spherical barrier, a hole in it would definitely open facing his back. However, there aren’t many heroes who possess the skill to pull off the tricky feat of aiming for Siegfried’s back while fighting him in the first place.