Tradition? Center of gravity? Novelty?
Most references I’ve seen trace back that shape to clubs, which were used as improvised targets before bowling was bowling. Following the brief heyday of candlepins bowling, the big bottle-shaped pins were favored because they score higher.
well, the traditional shape with a bulge in the lower portion gives them a low centre of gravity, making them fairly stable, plus the neck with the bulge on top makes it easy to grab them and set them up again (whether human hand or mechanical setter).