They're not leaving it as a secret. they had to stretch a storyline over three movies, y'all seem to have wanted JJ to cover everything in the first film. he had to leave topics open and relatively unexplored for the next two directors.
JJ didn't cover anything in the first movie. We barley had any idea what the hell was going on in the galaxy. The importance of starting a new trilogy is setting a strong foundation for the next directors to build off of.
We had 3 movies of a rebel alliance and after episode 7 we still don't even really understand what really happened to them? Were suppose to assume the Empire just faded away and returned as if they never left. Luke is just gone and apparently he started a new Jedi Academy? Maybe? And Kylo destroyed it because Snoke? Kinda? The Force has been gone but now its back? I think?
These are the MAJOR plot points of episode 7 and then didn't even occur or touch on in the movie.
That's the problem, they are trying to push forward where we don't even know what's going on now or in the past.
The original trilogy worked because it didn't dwell so much on what happened in the prequels or pre-episode 4. It was it's own self contained story. Even episode 4 was pretty standalone-ish.
Episode 7 is purely a setting based off of events that happened off screen. You can't even go back to episode 6 to see what might be going on because its changed so drastically.