If we are talking about the 80s when the Mountain bike essentially came to be "mainstream", then we need to remember all the factors of that time too. The economy, the fashion, the bicycle industry, the infancy of the Mountain bike, so on and so fourth.
Certainly I think cool, and Fashion played a huge part, as did the surrounding propaganda from newly launched publications dedicated to the Mountain bike, like Mountain Bike UK, Mountain Bike International, and imported American publications like Mountain bike Action. But this new product, form of transport, freedom bike if you will, was also getting book press too, example "the mountain bike Handbook", "the off road handbook", "Richards Bicycle Book", etc., etc., in an internet free world, this was our only resource, and of course it played its part perfectly.
Add into this, the fashion led advertising and subsequent marketing from Companies like Muddy Fox, with a bike named Courier, Images in the press, the Likes of Frank Bruno sat aboard his Explorer, etc. etc., the Mountain Bike was always going to be used as an inner city form of transport, and most would never see a mountain.
Thankfully, for the first time, it was looked upon as cool, a bike you didn't have to be fit to ride, anybody could ride one. The bike boom had begun.
Couriers/Messengers/Dispatch riders, don't get paid fortunes, its hard graft, its dangerous, they take there lives into there own hands every single day, breakages cost money, time cost money, traffic delays cost money, accidents cost money, and all in a world where the pay wasn't great to start with, and to be fair in that world it can be quite cut throat to ensure your the rider that gets the next job. Every extra Job, gets you more pay.
A Bicycle courier lives with his bike, his bike is his best friend, his prized possession, and his sole avenue of income. But they don't do it for just an income, they do it for the thrill of the chase, to ride the bike, to stay fit whilst getting paid.
And if they have this lifestyle, they are pretty serious cyclists, there whole world invested in there machine.
The Sport of Mountain bike racing blew up and quickly, anybody could race, no Cats, etc etc, at the beginning it was all amateur, turn up, and race, simple as that.
And for those that thought outside the box, if they could utilize the mountain bike for work, they could have the same bike for racing at the weekends. And that was it really, it really was, as simple as that.
With a well built frame, you could have the advantage of lower weight and higher strength, you could have the advantage of more powerful brakes, and as time progressed, stronger wheels, components etc.
Bearing in mind these guys rarely take notice of road signs, traffic lights buses, black cabs etc, they were/are against the clock to get the parcel or post delivered on time on deadline, Tagging busses and black cabs, feet firmly strapped in, taking shortcuts down flights of steps, hoping curbs and dodging pedestrians, all part of the grueling daily routine. The mountain bike lent itself to this very well.
But then the weekend came, and they could race the same machine, with just a minimal swop of tires/wheels.
I think the magazines capitalized on this, helping show this in images and short txt, gave the "Mountain Bike" a broader use. Inner city kids, dads commuting etc, all helped move metal.
Frame Builders were getting requests, and so it began.