All of the adult novels were considered cannon outside of a select few (this is pre-Disney).
I am sure the Empire outlawed even the mention of Jedi, further helping to wipe them from memory. Can't be a part of pop culture if the media is forbidden to talk about it, right? 30 years would feasibly be enough time for them to become virtually "forgotten". Since New Hope was primarily from Luke's point of view, you can see how he would have little knowledge of Jedi since his Uncle refused to even talk about them.
It is getting a little dicey with the Jedi still existing to a certain capacity in Rebels. Jedi need to all be dead except for Yoda and Obi-wan for the purge to have it's full effect.
We do need to see more from the post-Return of the Jedi world to help broaden the world of Force Awakens. I would also argue that this is more reason why we need to see the Knights of the Old Republic/Tales of the Jedi/Tales of the Sith times come back. Rumor has it they may make Knights of the Old Republic cannon soon, which would allow the world to breathe a bit more without rehashing the exact same characters from the movies.
In addition to all that about Palpatine controlling the media, let's not forget he was a super powerful witch with mind control powers to wash over people a spell of forgetfulness. Okay not literally that but you know what I mean, he had the whole senate under his Sith powers, its conceivable that he could have use his Sith powers to make people forget the Jedi.
Also Obi Wan said, "Darth Vader, who was a pupil of mine before he turned to evil, helped the Emperor hunt down and kill all the Jedi." meaning that it was well established the events happened in Obi Wan's lifetime. He was in his 60's at the time, so yeah 30 years seems about right.
To Stratos: yeah most of my comment was sarcasm but it was not in jest, I honestly do think Lucas was brilliant and that the haters are just butt hurt they realized the creator of their special magic story didn't have the same emotional attachment to it as they did. Hence why they use the phrase he raped their childhood, admitting they are butthurt by his actions.