When was the game up for prog rock?

That blows my half-baked theory out of the water then. 

Overall I don't think nothing in particular happened, just the "circle of life". What happened to glam metal at the end of the '80s? Punk at the end of the '70s? Grunge in the mid '90s? Psychedelic rock in the late '60s? Genres enjoy a period of popularity when they're new and then they lose popularity. One could argue prog had actually a pretty long run ('69-'78 roughly) and there were several succesfull acts after it as well (Marillion, Tool, Dream Theater). 

Basically bands were changing or breaking up as you say for the simple reason that those bands weren't making money. The end was coming when bands like Emerson Lake & Palmer were taking huge orchestras out on the road. They were bleeding money. You can't keep doing it without something breaking. The other thing is that people were getting sick of 10 minute guitar solos and 30 minute songs. Punk offered a simple alternative where you reacted to what the musicians were saying rather than what they were playing - mostly antiestablishment and a rejection of Maggie Thatcher's England. When prog revived in the 90's it came about because of the love of music of the prog era and the challenge of being able to play it. This era of prog isn't about money because there isn't money to be made from prog, just a love of the music.

Exactly, it seems the answers are debating that it is not dead, that's not the question. I don't think it is dead, but there certainly was a huge change that happened in the 80s that created a decline in the output of progressive rock, simple change. There was a period of bleakness in the late 80s thru most of the 90s, but it seems a lot of prog/proggy music has come back over the past 15 yrs or so. I still doubt the music masses know who the hell Gentle Giant or Camel is, and if they do they don't call it progressive music.

The core of the OP question to me is why did progressive music lose its importance after the 70s and what caused that change? As was mentioned there were many bands/albums that ranked in top lists, JTull issuing 2 #1 albums in the US and both charting way up there in other countries too......Would never happen today, or even in the 90's. What was it Crest Of A Knave that did really well from 1987 and beat out Metallica for a Grammy, not that I consider Crest a traditional progressive rock album but more straight up rock, proggy maybe at best. 

Lewian wrote: siLLy puPPy wrote:   The 80s was anomaly and a forced one by the recording industry. Actually one could say the seventies (or rather 67-76) were an anomaly with all this outpour of creativity and complexity (and, to some extent, escapism) that could gain popularity. Normally what appeals to the masses is more simplistic (I'm not again straight simple music though). One could argue that what happened afterwards "corrected" the "excesses" of the early seventies. I suspect the more natural place of prog and the prog lovers is a niche, not the focus of public attention, and a niche is where we are now.

Hmm, yes, good observation.