Not sure there is a why more of a why not. Don's pattern pack a has regular sling and the thumb hole pattern one. I had never seen on before. So I Made one for my youngest and then this one for my oldest. My oldest and I both liked the thumb hole and found it a nice addition. Is it needed. Nope. But I liked how you could hold it that way when the gun is on your shoulder.
Ps. I have no affiliation with Don Gonzales, but I do like the pattern pack and sure enjoy his you tube videos he shares a lot of great knowledge.