Since this has already been explained in here, this will be the reader’s digest version. The Empire was bigger than the Rebel Alliance. It was never a fair fight. After Sidious was overthrown the war continued on for a while but eventually the New Republic “won” and soon began the process of demilitarizing. They demilitarized in part because the Republic was never supposed to have a massive military and frankly the people were sick of war...it was Sidious that was behind that evil and the New Republic sought to undo what he had done. The First Order grew out of ashes of the Empire in secret. By the time the New Republic caught on to what was happening it was basically too late. A lot of the population was done fighting for the most part. The First Order, like the Empire before it, was and is a much larger military force than the small Resistance that Leia was able to put together. Yes, I may have taken some creative license in my readers digest summary but that is the jist of it. Feel free to read up on the back story if you like, but just as it was to start Star Wars, the opening crawl of TFA filled in enough back story to drop you into the story. If you want all the details, you are free to read the supplementary material. Maybe some day they will make a movie about the period after VI, but that is not the story they are telling with the current trilogy.