Eddie Fitzgerald recently asked "Why did '60s films look so bad?" This is a subject dear to my heart, since I've frequently stated that I think the '60s were the worst era for American studio films, at least from a technical standpoint.
As to why this was so, here are a few things I'll suggest as possibilities:
1) The collapse of the studio system, which started in the late '40s, finally became complete around this time. The days when every movie, even B-movies, could count on having great technicians in every department, were gone.
2) Studios became increasingly dependent on technical short cuts as they struggled to keep afloat. And these technical short cuts then became common practice even in big-budget movies. This isn't exactly related to the look of the film, but in the late '50s and early '60s many studios stopped recording dialogue on location: instead of sending out microphones and recording the lines as the actors spoke, they'd just dub in everything later. Robert Altman was appalled to find that at Universal in the '60s, the studio had a blanket policy of doing ADR dubbing on any scene that wasn't an interior. The studios were applying similar short cuts to lighting, camera setups, set design -- little things, but the essence of a great-looking movie is that it goes all out to get the best possible result even in the little things. This wasn't really happening any more.
3) And along with the short cuts, a new wastefulness in the sense that movies were free (and in the case of roadshow pictures, encouraged) to be longer and slower than ever before. In part there may have been an aesthetic decision that movies didn't need to be as tight and fast as they'd once been; Howard Hawks, whose movies slowed to a crawl around this time, said that he decided that in the age of television, audiences wanted to take their time and spend time with characters, rather than see a closely-argued story. The pace of movies, including movie comedies, became very slow and stodgy around this time.
4) The influx of live television people, many of whom were very good but most of whom didn't have the "painterly" conception of movies that the old guard had. Live television is a great training ground for working with actors and under pressure, not so much for lighting and composition. The end of the '60s and the early '70s saw a new influx of people who had been trained in movie-making -- either film school or, even better, working for low-budget guys like Roger Corman -- and that's one explanation of how '70s movies were a bit better-composed and more interestingly lit even though U.S. studio technical standards were still pretty low.
5) Increased consciousness of the fact that movies were going to be shown on television, which might have had something to do with some of the more drab choices of composition and lighting, especially for movies that weren't in widescreen.
In terms of color, lighting, composition and special effects, England and Italy were way ahead of America in the '60s, although they weren't fully able to turn this to the advantage of their own film industries. Instead English and Italian studios provided their technical expertise to American-made or American-financed movies; films like The Pink Panther -- shot in Rome -- or Dr. Strangelove -- shot in England -- looked much better than the stuff being turned out by U.S. production factories.