Because the rocker trim overlaps the front fender and the rear quarters, you are able to use a bolt on clip (clip slides on trim, bolt goes through the fender/quarter, then nut on the backside). The center part (the actual rocker) is solid on the back and underside, so there's no access for a bolt. The center clips are riveted to the body, then the trim snaps on the clips. That's the reason they are so hard to find in good condition, once you pull the trim off the little tabs on the clips usually break, rendering them useless. Also, having the ends bolted on helps them from being accidentally torn off.
Anton