The real question is “Why hasn’t Progressive Rock ever made a comeback?”. All other popular genres from the seventies do have periodic revivals. But not ‘prog’. We’ll get to they ‘why’ in a second.

But to your question: the primary reason progressive rock is the Rodney Dangerfield of all the big rock genres? The critics LOATHED it. From all the jokes and contempt, most young people are surprised to learn just how big these bands were in the ’70s. But most young people also cannot appreciate how influential ‘rock journalism’ was during rock’s heyday. Rolling Stone, Creem, et al. mattered. And the critics hated everything about Progressive Rock from day one.

What matters above all things to most pop music critics is something referred to by musicologists as ‘authenticity’. A one word definition for ‘authenticity’ might be ‘honest’. And the qualities necessary to appear as honest strongly favour music which is both musically and lyrically stripped and free of any artifice. Americana? Good. Can’t get much more authentic than songs with a strong folk influence. In fact anything labelled ‘Roots’ is a plus. World music is the ultimate in authenticity because it’s the music of the people. Punk? Also good because it’s so raw and primal. Even dance music is now grudgingly accepted as good no matter how artificial it may seem because it has one simple (and honest) goal: to get people dancing (the most primal use of music)! The one thing all these musics have in common is that they are not aspiring to be more than what they are.

But anything that becomes too ‘advanced’ either lyrically or musically is almost automatically suspect. Rock music is always in danger of becoming ‘inauthentic’ when it aspires to be something more. Then, it’s considered ‘over-produced’ and worst of all, ‘pretentious’. So from the critical viewpoint Progressive Rock was about the least ‘authentic’ music one could imagine because it aspires not just to include elements of classical or other advanced musics, but to actually be more advanced. Great Progressive Rock demands to be considered as something more than a rock song. And generally, critics won’t stand for that.

Now there have been some critically appreciated bands such as David Bowie and Roxy Music which are not ‘authentic’ by any means. Musicologists use the technical term ‘artifice’ to describe art which is consciously pretentious. These artists are consciously trying to be fashionable and ironic. Progressive rock was not trying to be ‘fashionable’ or pop. It was aspiring to be real, unironic art.

Worse than the musical aspirations, bands like Yes and Rush often had frankly adolescent or just plain nonsensical lyrics. And that drove critics apoplectic. (This is a shame because some bands like Gentle Giant and Genesis, could write especially great poetry.) Now lots of bands have silly lyrics, but when Led Zeppelin sings about a ‘tadpole in a jar’ or a ‘bustle in yer hedgerow’ it’s cool because, hey, they’re just having fun. Critics would say that they were really a super-charged blues band. Nudge, nudge. Wink. Wink.

In any case, you can’t overestimate how critical loathing affects this whole discussion. When Progressive Rock fell out of favour, it fell harder than other styles because there were a whole lot of people rooting for it to fail.

And in that respect the bands themselves did not disappoint. Most of the big groups had two or three albums of good ideas and then just petered out. That’s a pretty typical shelf-life for most genres of music, but groups like Yes and Emerson, Lake and Palmer became a bit of a double-joke, buying castles and so forth and having public rows even as their music was in decline. Such things can be forgiven if the music remains at a high level, but such was not the case. To put it another way, This Is Spinal Tap was as much about Emerson Lake & Palmer as Nigel Tufnel and David St. Hubbins.

One other diversion: The contempt is so complete that even the name ‘Progressive Rock’ is no longer in common usage, having been replaced by the nickname ‘prog’. My guess is that this is an attempt at being self-deprecating. As if to say “Hey we get it: even the name was pretentious! So we changed it because we don’t take ourselves too seriously, folks. Really” But frankly, it’s kinda hard to take any genre seriously with a nickname that rhymes with ‘frog’.

Now lots of genres have a period out in the cold. It’s easy to forget that disco was reviled for a decade. Here is the unique thing about progressive rock among the whole history of older genres: Most genres as popular as Progressive Rock was have periodic resurgences. Motown, Disco, Rockabilly, all have made comebacks. Even ‘jam bands’ with very long pieces have big audiences. But not progressive rock.

This partly has to do with the fact that it requires a very high level of skill to do well. And frankly most bands that do try to create updated versions are simply not very good musicians or songwriters. If you check out retro groups in any other genre they do tend to be very good and in fact expand and even re-invent their respective genres. But even hardcore Progressive Rock fans agree that almost every new prog band (with one or two very notable exceptions) sound nowhere near as good as ‘the originals’. So the jokes about endlessly pretentious songs just keep be proven true over and over again.

The other side of the coin is that most audiences these days simply don’t have the attention span for any complex music. All the popular musics tend to be ever more simplistic. Jam Band music might seem to be an exception, but it’s largely a hypnotic genre, consisting mainly of very long solos over very simple (as in modal) chord structures. Spinal Tap was also mocking these sorts of groups in the scene where they re-invent themselves as “Jazz Odyssey”.

In summary, Progressive Rock was the product of a unique moment in pop music history when there were audiences for long, complex pieces. It was very popular, but the critics hated it. Since then there has been a fundamental change in the way we play and listen to music and in our now seemingly permanent desire for more simplistic styles. Both these forces have prevented Progressive Rock from making a comeback. So the jokes have metastasized into what appears to be a permanent state. In short, Progressive Rock is the one rock genre that’s easy to make fun of because it has no one to defend it in the current wider culture.