I can not place the timing of the trend from walking off to rapping off but I imagine it coincided roughly with the placement of bolted rap anchors circa 2000, which accelerated the loss of downclimbing skills among new climbers, as well as shifting the trend for those uncomfortable downsoloing with 150' of air from using walkoffs (down Roger's) to raps, and from walking 3 minutes to a rap (High E) to looking for a rap within 50 yards.
The bolted rap anchors, for the most part, replaced makeshift anchors already quite established. The bolts accelerated trends like top-roping and the "single-pitching" of formerly multipitch climbs, but rapping rather than walking or downclimbing was well-established before the bolts, although it too became even more common once the bolts were in.
My theory is that the real impetus for rap descents was the banning of parking on 44-55. It used to be easy to return to your car in between climbs by walking back to the Uberfall. Hardly anyone carried a pack of gear to the base of anything. The removal of parking to remote locations made it necessary to carry your stuff to the base, and once that happened, returning to the base as quickly and efficiently as possible made rapping the method of choice.