On a jig you would want the hook to stick the fish in the roof of the mouth, not "slide" forward to the lip or corner of the mouth. Circle hooks are not intended for rod jerk hooksets, but simply "pulling" forward on the line (or simply reeling) to bring the hook point into contact with the fish. A circle hook has a hook point that is bent inwards towards the shank, which is contrary to what you would want on a jig because it wouldn't engage with the fish's mouth until it came to a lip or corner ... and then there's the possibility that the lead head of the jig would interfere with the hook sticking into the fish's mouth.