In the early innings you don't necessarily know how many runs you'll need. Bunting then seems bad in general. If you have your ace on the mound, maybe you have a better idea of that number, even early. Or, if you have a good OBP offense with poor slugging, maybe bunting can help you wring a few more runs out, by cutting down on the two-runner innings that don't produce anything. But I'm laying out extreme cases here; in the average game the early sacrifice is so far from optimal that it's hardly worth looking at anymore.