Hi Dave. I’m sure others will correct me, but I think it is related to the advent of boom stowed poles and the need for the pole to be at the right height to be stowed parallel to the boom without additional adjustment every time the kite hoisted and dropped. I think it is far easier to achieve this with the elastic below the pole (usually zig-zagged across the width of the boat behind the front tank). It does introduce the challenge of preventing the pole from bouncing on close reaches, but the benefits of boom stowed poles are significant, not least the fact that they’re attached to the boat!
The arrangement described with the elastic take up under the deck is much older than you suggest I had a system under the foredeck on GP 10444 which I rigged with a boom stored spinnaker pole. Other boats I have owned since then have all had an under deck take up system some with the pole stored in the boat clipped on by the crew to a hook on the uphsul/ downhaul and some with the pole stored on the boom mainly newer boats. On my current boat we have added a shock cord loop on the mast under which we tuck the pole to stop it flapping about when it is stored on the boom.