It has to do with how the fibers move in each one and how much a bend would derate it by. In a rope when the fibers are woven together the movement of the individual fibers along the entire length of the rope are less pronounced. The weave makes the fibers work together locally, and so bending does not reduce the individual fiber strength as much. In a poly sling the fibers need to freely move along the entire "length" of the sling as they loop around to equalize the force amongst all fibers. So they are working together globally, but not locally. This means that a bend or knot will reduce the strength much more than the rope because you are halting the movement of the individual fibers.
I hope that makes sense.
I hope that makes sense.