There are a lot of reasons for that; in addition to that being part of the underlying culture of the post-Vietnam War, post-Baby Boom era, there were also changes in film stock that made everything darker and grainier whether it suited the subject matter or not. And the MPAA ratings system made it possible for filmmakers to let loose with things they didn't use to be able to show before. Also, now that all new TV was in color, filmmakers wanted to do something to stylistically differentiate theatrical films from it. I'm convinced Richard Lester wanted it not to be obviously stylistically derivative of the most famous Errol Flynn version and the then-recent Disney animated version.
Columbia also made Taxi Driver the same year while Jodie Foster supplemented that with Freaky Friday and Bugsy Malone at Disney (the one studio that tried to hold out from all that until their resistance broke by the end of the decade), and Paramount, respectively. This was Hollywood, 1976 style. The last year before Star Wars changed everything.