Likely because most crossbow arrows are commercially made and purchase already assembled. The arrow makers tend to use 4" vanes which are not that compatible with helical application. It's largely us crossbow loonies that keep doing different, new and improved things: high FOC, short vanes, helical application. E.g. I had self-made string stoppers on my bows a full year and a half before Parker and Excal even had them available for sale; used custom made aluminum moon nocks half yr before Gold Tip came out with theirs.