I dont think Star Wars deviated that much from the general mood of the 70s. It was experimental cinema in its day--FOX had planned Damnation Alley as its major sci fi film--SW was a little side project.
SW took place under a war and galactic dictatorship, lots of people died, and the bad guy lived to fight another day. Vietnam and the hippies (the Force, Chewbacca ) are echoed in the story. R2D2 represents the technological comfort that sprang up with innovations in home appliances (I just made that up but I think it has some truth. lol)
It may have been influenced by serial adventures but it wasnt a straight update of them(the same is true of Raiders--how many 1940s serials had a main character with a Polanski-like rape scandal?).
Audiences change too--the people who wanted to see foreign and alternative stories in the 60s gave way to a different audience by the late 70s.
Although it probably killed serious science fiction films. Before SW the genre was geared at adults for the most part. Silent Running, Colossus the Forbin project, Planet of the Apes, 2001, Forbidden Planet, Day the Earth Stood Still...After Blade Runner there hasnt been much in that vein from the major studios...
The main change after SW was the multi national corporations buying the studios and the focus on merchandising.
I think its possible that the prequels were a more obvious damage to moviemaking--since they started appearing there was a noticeable shift towards franchises and a lapse in even the most basic attempt at character and dialogue. Coincidence? I wonder. Maybe the studios thought: hey, people are going to see these things no matter how bad they are...
Star Wars was not deep in either department but is practically Shakespearean compared to a Transformers or Clash of the Titans 2010. ROTJ despite its flaws is more emotionally involving than most of the big studio movies I watch these days.
Cameron isnt getting the same criticism because there isnt anything but franchise, remakes and comic book movies from the studios, and the critics working now grew up after Star Wars.