The low end road bikes back in the 70's and 80's were horrible, most of them ended up with gears and chains rusted in place and cables frozen in their housings. They ended up unsafe and unused. When Mountain bikes were introduced they offered a relatively practical vehicle that was more weather-proof and safer to ride for most people. Mountain bikes could be ridden off road (gravel shoulders) on uneven terrain safely, in an upright, comfortable (for most non-riders) position.
I believe mountain bikes made cycling more accessible and easier for a wider range of people.
Lots of road riders got into the sport via mountain biking, it was a natural progression when they started riding on the road and realized fat tires just don't cut it on pavement.
Cycling as a sport has had many booms and busts over the last century, the latest road bike boom in America can probably be attributed to non other than Lance. Watch what happens if he wins the TDF next year.