IMHO-
I think the seemingly new prevalence of gravel bikes and gravel rides has a lot to do with the cycling industry's continually evolving marketing strategies. Sometime in the nineties, the industry went to performance road bikes that offered little or no extra space for tires larger than 23mm. If you owned a road bike (steel of course) before 1990, chances are you could put 28's, 30's or even 32's on it, and ride just about anywhere you wanted. My 86 Ross Centaur traveled hundreds of miles on south Ga. dirt roads with 32's. Cyclist that came on board in the nineties or later were relegated to road bikes that came with 23's and perhaps didn't even provide room for 25's. So, for many of us it's a return to an earlier time when bikes were more versatile, and for others, it's all new stuff- a bike with larger tires and accommodating brakes that will go just about anywhere. Anytime the industry can "develop" a new style of bike for the market with new (maybe better) products, it's good for business and they certainly have jumped on the gravel band wagon when in reality, they have already been there.