A mountain bike started off as little more than a reinforced road frame. Of course this was back in the 60’s when the cruiser bike featured a large, cushy seat and a laid back rider position. The first mountain bikes had reinforced welding at the joints, and not much else. They rode as roughly on trails as a street bike, and didn’t have a suspension.
But they didn’t fall apart on the trail, and that was a big first step in the evolution of the mountain bike. Once you had a platform that was stable, it was much easier to improve on than the road bike was. Today, much of the innovation in the cycling world is driven by mountain bikes.
Take suspensions for example. Before the mountain bike, shock absorbers on a bicycle would be an absurd gesture. Now, not only are most front forks built with shocks, but you can find full suspension bikes in any respectable shop. All thanks to people who looked at a bike and said “I’d like to see what that does against rocks and mud.”