Then the synthetic materials came along - especially after WWII. Fiberglass proved to be an excellent rod making material, & by the 1960's had replaced bamboo as the main rod material. Being lighter & cheaper to produce on a large scale, fiberglass was was able to meet the post-war demand of the growing popularity of fishing in America.
In the early 1980's (maybe the very late 70's) graphite rods came along. These proved to be lighter in weight & more durable than bamboo or fiberglass. At first they were more expensive then glass rods, but as improvements were made in both the materials & the manufacturing processes, graphite rods lowered in price & rose in demand. They became much more available on the market & soon replaced fiberglass as the main rod material.
Because of the nature of graphite as a rod material longer rods made for lighter fly lines became a reality. Likewise, rods for heavier lines became physically lighter & much easier to cast - especially one handed. Truly fast & extra-fast were now a possiblity for fly rods like never before. This began a trend with fly rods of faster, stiffer rods. Soon the majority of new fly rods were fast action, where the only flexing was in the upper third of the rod, often even closer to the rod tip. These faster rods made it so much easier to generate a faster line speed which meant (in theory) that casting further distances with a tighter loop was easier than before.
To me this is a very good trend. Please don't get me wrong - there always were graphite rods being made that flexed a bit and had a more relaxed feel to them, its just that they were rare &, I think, not the norm for the time. I'm glad to see that there's a lot more variety in graphite fly rods these days, giving anglers more choices. To have more rods on the market with a desirable, pleasant action to them with the benefits of graphite is something I like to see. As manufacturers like Scott, Thomas & Thomas, and others design more bend into their fly rods, more anglers can experience other rod actions. That can only be a good thing.
Fast action graphite fly rods certainly have their place, but for almost an entire generation of anglers, it was all many of them were ever able to experience. Now that this is changing I can only see this current trend enhancing the sport. In rod action, what's old is new again, I guess!