Scouter, I must reply.
Why a sling and not a bipod?
A sling is more flexible. For instance, one can use a sling from the standing, kneeling or sitting position if prone is not available (tall grass, broken terrain, swampy or whatever...) Even in the prone position, a sling is easier to adjust than a bipod. And lateral movement is just as easy. Shooting on a downward slope from prone is particularly inconvenient with a bipod.
When not in use, a bipod is more factors to tangle, bump or otherwise emcumber movement. Brush, window ledges and door frames all are mysteriously attracted to bipods. Scopes, too, for that matter.
I have carried rifles by slings. I have carried rifles by bipods. Slings are better.
I do have a bipod that I switch between a couple of my rifles. It does serve well, IF I have a proper shooting position. For a quickly assumed position, a sling serves me better. Even with a bipod, I still use a sling.
If your results differ, good for you. You did ask why a sling instead of a bipod.