What is going on with that guy's bike. It is like the Canyonero XL Extreme of bikes. What kind of riding is it suited to? You wouldn't pop down to the grocer in a Unimog, right?
At the time, the perception was that mountain bikes were just more rugged and comfortable to ride. There was a not inconsequential amount of marketing hype about the condition of roads, and the roads in some areas were in fact pretty crappy. Just due to market timing, MTB's were also where most people first experienced indexed shifting, which did improve the ease of riding a bike.
In the case of e-bikes, there's a tendency to get "more bike than you really need" because the marginal cost is acceptable and the motor takes care of the added weight. And from a pragmatic standpoint, the difference may also be mostly aesthetic.