Prog was mainstream in the late 60's and early 70's as the music industry tended to be run by muisc fans. That changed in the late 70's and big business took over. Luckily for us the the internet happened and it is much easier to get information about non mainstream musical styles. The beauty is the corporates have a hard controling what happens on the net. The 80's were pretty dire as far as trying get information on new bands etc. but somehow I discovered some new stuff at the time through friends and mainly through magazines like Guitar Player as they are less driven by commercial tastes and focus on performance and creative ability. It's not just prog that is locked out of commercial radio/tv these days. Steve Lukather of Toto (and they were huge) said that record companies wont sign a rock band over 30 years old and the radio wont play them. I recently missed their tour of Australia because there was no press at all. Same almost happened when Yes came out, I only caught them because their tour was postponed.
I'd love to see Prog and Fusion and Acid Jazz and etc etc etc more well known and broadcast but I don't think we'll ever see what tends to be more complex music going mainstrem because to most people music is background noise or the soundtrack to other activities. Music like Prog, Fusion, Bebop etc requires active listening and it's just not that important to most people.
I take pleasure when I find someone I can "corrupt" to the fold and they are about. I can play music at work and I've educated a couple of people and turned them on to the real alternative music.
There will always be ebb and flow in the profile of different styles.
There's people out there saying "Man, wouldn't it be great if Death Metal became the mainstream music". Thank go it wont.