The choice of N. gibbosulus in Mediterranean web sites and N. kraussianus is South African sites is often a purpose of their availability in the vicinity with the web sites, but would not clarify why this genus was most popular in excess of other shells of similar dimensions and shape. https://medium.com/@spoilmevintagediy/the-rich-history-of-beadwork-a-journey-through-the-ages-4e4fd2e474cf