The sport of Motocross is a niche sport that originated in Belgium, and was originally called Scrambles, The sport spread like wildfire all throughout the United States after being introduced to America by a visionary entrepreneur, (Edison Dye)
He had seen the way dirt bikes were raced on motocross tracks in Europe which was different than the dirt bike racing at the time in the USA, we had desert racing and some cross country off road racing but motocross on tracks was a new concept that wasn't yet being done in the USA. He had the vision and belief that it would it be a big hit over here and he came up with the idea to be a Husqvarna distributor and to help grow the sport in the USA, The Husqvarna was a high quality purpose built Motocross bike that wasn't available at the time in the USA, he enlisted the help of Top European Motocross racers, Torstan Hallman, Roger Decoster, Lars Larson and several others he also teamed up with a young man from California Mark Blackwell, he brought him to Europe to learn as much as he could about the sport of motocross and to learn how it was done there to help bring back that knowledge and help introduce the sport to America.
When they first came to America they traveled to all different states all around America going to motorcycle shops & doing exhibition races to show the sport to people, they helped make tracks, and explained the rules to people they got publicity by going to Television & radio stations to explain the sport of motocross to the public, and to market their exhibition events.
The early 1970's was like a perfect storm of things all caming together all happening in unison all across the country.
The movie 'ON ANY SUNDAY" Came out and was a big hit, paying in theatres, the movie even had Star power with "Steve McQueen' a very famous actor at the time, The movie is widely credited to have helped the sport of motocross and all forms of dirt bike riding grow all across the country.
Also in the early 1970's time period there there started to be many purpose built Dirt bikes made specifically for motocross and off road for the first time ever, including mini dirt bikes, Like the Honda MR50 & XR75 and other kids and adult models that were affordable for most middle class families at that time.
Even Harley Davidson and Indian motorcycles and many other brands started making mini dirt bikes in the 1970"s.
Then in the early 1970's they had the Super bowl of motocross in Daytona, and a new pro national motocross series hit the scene, and motocross started booming, before long there was amatuer motocross races in every state.