Gravel racing has always been a thing but was a bit more on the counter-culture side of cycling (much like ultra-marathons). It’s become more serious/ professional, in part to “retired” world tour riders seeking them as a fun challenging day on the bike, and also the new way sponsorships are social media-focused. With a more competitive top level of competition, and the ability to compete alongside them, the popularity exploded. I’d say it’s more like road marathoners doing trail 50-100k ultras. World tour and elite MTB racers are putting in 30+ hr weeks in the saddle.
I’d also like to think gravel racing has taken on popularity because it gets riders off busy roads for training and racing. Everyone from retired pros with families to the casual weekend warrior likes that.