The way I understand it (or the way I suppose it is) is a combination of two things:
(1) Joss Whedon really loved that show and was very hurt and put out with the way that Fox handled it. I think he kept his hopes up, like many of the fans did, that one day the show would air again and he would be able to finish the story.
(2) A lot of the story is being finished through the comic books now. Granted, it's not the way the fans preferred to get it, but it's still a way. The process is just longer.