regarding loveless's question: why pop music became less experimental around 1968 (about the time Music from Big Pink and John Wesley Harding came out) and why pop music started to retreat into the ``back-to-nature/back-to-the-country'' Walden-esque bean patch.
Two comments, the first being a minor quibble, the second being a more substantive attempt to address the question.
First, one assumption of the question (at least the way I've condensed it) is that Music from Big Pink is less experimental. They may well show just how powerful album covers are in determining how we think about an album. I'm talking about the utterly homely pictures showing ``next of kin'', etc. on the obverse. But if you listen to especially the vocal harmonies, you can see that there really is something very experimental and ground-breaking about this album. What do I mean by this? I even hesitate to call these vocals, vocal harmonies because the point isn't, I think, to create a chord, although if that happens, all well and good. The point is to have two or three semi-independent voices calling out when and where they please--counterpoint in other words. It's an old concept, but until Music from Big Pink, I don't think it had been used all that much in pop music. Sure, to a certain extent you have call and response in gospel music, but that's not simultaneous independent voices, and that's not really counterpoint. The idea of treating voices like horns in New Orleans jazz band isn't a new form of counterpoint (just look at King Oliver's band), but doing this within pop music, I think was new, and for the most part, bands have not extended or even explored these kinds of experiments.
To address the larger claim--that music (in this case not from Big Pink) became less experimental after 1968, there's one reason that I don't think has been mentioned thus far, which has to do with just how experimental pop music can be and still have a reasonably large audience. This isn't meant to be a semantic argument--that is, that music without a large audience can't be called pop music. It's an economic argument, namely that with very rare exceptions, bands knew that there was a price to be paid for being too experimental and that price was that one would lose much of the audience that one had built up. And with losing that audience, one would lose the record company contract and then the opportunity to get one's music out to a general public.
We might think now of the late sixties as being a very adventurous time and record companies being quite willing to let bands experiment however they wanted to (I do think there was more latitude then than at other times), but it would be good to keep in mind that you also had scenarios such as The Zombies struggling to get their last album mixed down and paying for that with their own money.
My point being that certain types of experimentation by certain groups were permitted. However, a great deal of experimentation, especially if that experimentation happened to eat up a lot of studio time, wasn't something that record companies were all that happy with.
I'm suggesting a slightly different argument here than I originally started with, so let me just work this one out before I get to the main line of thought. It may be that record companies started concluding after a certain point (1968 or so) that the sort of experimentation that they had permitted bands to test out in the studio, while it had had some positive effect on album sales, hadn't had enough of an effect to warrant continuing this line of inquiry. In other words, financial support for this sort of experimentation from the parent company was not nearly as forthcoming. (I frankly admit here that this is total speculation on my part. I somewhat promised cheepniz that I would go with a looser style of writing at times, and this is an example, although, of course, the final responsibility lies with me not c.)
To get back to the main point I was making, though, which was also an economic point, I think that bands may have understood that becoming too experimental would result in losing much of their audience.
Let's take a brief look at similar things that had happened in other fields. Classical music went through a real crisis about 1910-1920. Schoenberg's solution to propose twelve tone serialism has had many consequences. One of these was to make a firm break with the 19th century. Another consequence was to make modern classical music something that the general classical music listener didn't want to listen to. This is a problem that classical music still has to confront and that it still hasn't solved. In fact, I would say that the situation gets more and more dire. I don't think it would be an overstatement to question whether classical music as we know it, as it fits into the tradition of classical music from 1670 to 1910 as a format with a reasonably large, well-informed audience is essentially a dead and will not reappear.
Jazz provides another good point of comparison. After Parker solved the two reigning open problems that had been besetting jazz from let's say, 1920-1943 (namely, how fast can one swing, how much can one extend triadic harmony), jazz really splintered and went into different directions Mingus explored how musicians should try to communicate with each other more directly in their solos. Interestingly Bill Evans explored this also. Davis dispensed with showtune based chords and went modal. Art Blakey integrated other forms of African-American music into bop. Eric Dolphy with Mingus asked how conversational one can make one's playing, literally conversational in the sense that one evokes speech (see Mingus and Dolphy's cover of ``What is this thing called Love?'') . Coltrane initially started by asking how rapidly one can modulate (``Moment's Notice'' and ``Giant Steps'') but then left these technical exercises for a very different form of music that tried to combine modal playing, musicians playing at the same time, and more overtly spiritual and political texts as starting points.
This is where we are with something like ``Ascension''.
God help me on this, but I have never been able to sit through all 42 minutes of this piece (either edition). Some parts of it I can handle, and some parts I find can illuminate the kinds of things Townshend was trying to do in Quadrophenia, but it isn't easy to get through this stuff.
It wasn't easy for people to get through this stuff in the mid 60s either, and the upshot of that was that Coltrane lost a great deal of his audience. But it wasn't just Coltrane. Jazz, or perhaps I should say, jazz that explored new territory, became much more of an underground music after 1964 or so. True, you could always get ahold of jazz that retreaded similar ground to what had been done in from 1945-1960, but jazz that tried to experiment and try something new became a smaller and smaller subset of what jazz was in total--much in the same way that classical music that tried to do something new became a much smaller subset of all classical music produced and recorded in a given year as the 20th century went on.
The point I'm trying to make through these comparisons is that there may be some large-scale general forces at work in which the people who make this music understand that at a certain point they make have to make a decision between having an audience and reducing the amount of experimentation, and losing that audience.
The argument I'm making in other words is that certain forms may have built into them a certain threshold of just how much experimentation they permit and still maintain a large audience. (Of course, I admit this is all speculation on my part. I don't know where, if at all, that threshold is).
Stated in this way, it seems fairly clear why pop music might, after a period of experimentation, decide to step back for a bit. Now, precisely why this happened in 1968 I don't know.
Slight editings to correct spelling errors.