the adjustable suspensions are just easier to adjust, a bowline is a good knot, but i often find i set up the hammock and then realize i need to tighten or loosen one end by just a few inches, that would mean untying and then re-tying the knot, with the adj webbing, it's much quicker. clipping around the tree with the biner is nice and quick too.
Ring buckles are my preferred method to tie off with.... Easy for micro adjustments and the aluminum rings weigh such a small amount.
What I lack in knowledge I MORE than make up for with opinions.
I think to answer the original Posters question...Reason is we like to mess with stuff that aint broken. But honestly it is all about choice, like gargoyle said we like to try out new stuff and one thing clicks with you ...you keep on keepin on.
For me +1 on the Whoo-pie Sling. Love those things man. Plus had a blast makin them.